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Originally published In Press as doi:10.1074/jbc.M511101200 on February 6, 2006

J. Biol. Chem., Vol. 281, Issue 14, 9633-9640, April 7, 2006
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A Non-cleavable UmuD Variant That Acts as a UmuD' Mimic*Formula

Penny J. Beuning{ddagger}, Sharotka M. Simon{ddagger}1, Adam Zemla§, Daniel Barsky, and Graham C. Walker{ddagger}2

From the {ddagger}Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 and the § Computations Directorate and Biosciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550

Received for publication, October 12, 2005 , and in revised form, January 31, 2006.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
UmuD2 cleaves and removes its N-terminal 24 amino acids to form UmuD'2, which activates UmuC for its role in UV-induced mutagenesis in Escherichia coli. Cells with a non-cleavable UmuD exhibit essentially no UV-induced mutagenesis and are hypersensitive to killing by UV light. UmuD binds to the beta processivity clamp ("beta") of the replicative DNA polymerase, pol III. A possible beta-binding motif has been predicted in the same region of UmuD shown to be important for its interaction with beta. We performed alanine-scanning mutagenesis of this motif (14TFPLF18) in UmuD and found that it has a moderate influence on UV-induced mutagenesis but is required for the cold-sensitive phenotype caused by elevated levels of wild-type UmuD and UmuC. Surprisingly, the wild-type and the beta-binding motif variant bind to beta with similar Kd values as determined by changes in tryptophan fluorescence. However, these data also imply that the single tryptophan in beta is in strikingly different environments in the presence of the wild-type versus the variant UmuD proteins, suggesting a distinct change in some aspect of the interaction with little change in its strength. Despite the fact that this novel UmuD variant is non-cleavable, we find that cells harboring it display phenotypes more consistent with the cleaved form UmuD', such as resistance to killing by UV light and failure to exhibit the cold-sensitive phenotype. Cross-linking and chemical modification experiments indicate that the N-terminal arms of the UmuD variant are less likely to be bound to the globular domain than those of the wild-type, which may be the mechanism by which this UmuD variant acts as a UmuD' mimic.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The umuDC gene products are induced as part of the SOS response and are responsible for much of the UV-induced mutagenesis in Escherichia coli (1). These gene products are subject to an elaborate set of controls that regulate their activity (1). The LexA repressor provides transcriptional control, and there are several proteolytic controls on both the umuD and umuC gene products (1). The homodimeric protein UmuD2 is the predominant species during the first about 20-30 min after SOS induction (2). UmuD2, together with UmuC, plays a role in a DNA damage checkpoint, decreasing the rate of DNA synthesis and allowing time for accurate repair processes to act (2). This correlates with the cold-sensitive phenotype observed under conditions of overexpression of the umuDC gene products (2, 3). As the SOS response proceeds, UmuD2 binds the RecA/ssDNA3 nucleoprotein filament. This stimulates the latent ability of UmuD2 to convert to UmuD'2 by cleaving off its N-terminal 24 amino acids, resulting in UmuD'2 becoming the predominant species. The RecA/ssDNA nucleoprotein filament serves to bring together the active site dyad residues Ser60 and Lys97 facilitating deprotonation of Ser60 by Lys97 (4). The activated Ser nucleophile then cleaves the peptide bond between Cys24 and Gly25 of UmuD2 (1).

The wealth of structural data and models available for UmuD2 and UmuD'2 provide insight into how the two forms of the umuD gene products engage in multiple highly specific interactions (Fig. 1) (4-8), including with the {alpha}, beta, and {epsilon} subunits of the replicative polymerase, pol III (9). Of the two forms, UmuD2 interacts more strongly with the beta processivity clamp (also referred to as beta or the beta clamp) than does UmuD'2 (9, 10). In full-length UmuD2, the 39-amino acid N-terminal arms are stably bound to the globular C-terminal domain (4, 7) and form a distinct interaction surface. In the cleaved form of the protein, UmuD'2, the remaining approximately 15 amino acids at the N terminus appear unbound from the body of the protein and solvent-exposed (5, 6), revealing the buried portion of the C-terminal globular domain (4, 7). A series of truncations at the N-terminal arm of UmuD2 indicates that the first eight amino acids of UmuD2 are dispensable for the UmuD2-beta interaction, whereas deleting residues 2-18 results in a substantial decrease in, but not a complete loss of, cross-linking efficiency with the beta clamp (10). Thus, the umuD gene products interact with the beta clamp via both the N-terminal arms of UmuD2 and the globular domain of UmuD2 and UmuD'2 (10). This differential interaction appears to control, at least in part, whether the umuDC gene products act as part of a DNA damage checkpoint or as a translesion polymerase (9, 10). These interactions with the beta clamp are of particular interest because sliding clamps play a key role in coordinating the multiple DNA polymerases present in cells (11-15). The eukaryotic DNA sliding clamp proliferating cell nuclear antigen interacts with multiple proteins. These interactions are in part regulated by covalent modification of proliferating cell nuclear antigen with monoubiquitin or the small ubiquitin-like modifier, (SUMO) which is distinct from the role of polyubiquitination in proteolytic degradation (16-19). Duzen et al. (20) have suggested that UmuD2 and UmuD'2 play conceptually similar roles in modulating the various clamp interactions.

A version of the canonical beta clamp binding motif found in eubacterial polymerases as well as other proteins involved in DNA metabolism was postulated to be present in UmuD at residues 14-18 (Fig. 2) (21). A yeast two-hybrid experiment with the motif of UmuD showed, however, that these five amino acids are not sufficient for the interaction with the beta clamp (21). Given the fact that this result was obtained utilizing only the five-amino acid motif in UmuD, and cross-linking experiments showed that the region of UmuD between residues 9 and 19 is important for interactions with the beta clamp (10), we undertook a site-directed mutagenesis analysis of this motif. These studies led to the unexpected discovery of a new class of UmuD variant proteins that fail to undergo cleavage but whose properties resemble those of the cleaved version, UmuD'.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Homology Model of UmuD—The models of the UmuD homodimer were created by the combined use of the program LGA (22) for protein structure comparison and superposition and the AS2TS program (23) for homology model building. An initial model of the UmuD monomer (single chain) was constructed based on the crystal structure of UmuD' (Protein Data Bank code 1ay9, chain A) (24). The missing N-terminal arm was modeled by the LGA loop building/grafting procedure (22), using mainly the arm conformation in 1jhh_A, from the x-ray crystallographic structure of LexA (25) as well as the other LexA structures (1jhc, 1jhe, and 1jhf) as template structures to guide the local and overall conformation. In the final alignment (Fig. S1), UmuD Asp20 had to be inserted into the LexA template (between residues 80 and 81 of LexA), and this was done by the LGA loop building procedure (22). Finally, residues 1-14 are in an extended conformation (i.e. we make no prediction as to the placement of these residues; they are only modeled in a formal sense).

In creating the full UmuD homodimeric complex, we used LGA to superimpose our monomer model onto each of the template chains in the NMR structure of the UmuD' homodimer (4) (1i4v, chains A and B, model 1), but some minor clashes occurred that were alleviated by following the LexA homodimer instead (25). This procedure creates a cis (non-domain swapped) conformation of the UmuD homodimer. Because there is a very small "shoulder" region at the top of the arms, the trans UmuD homodimer model could be constructed from the cis UmuD homodimer model by swapping the arms as follows: the first 39 residues in chain A of our trans model were taken from the chain B of the cis model, and vice versa. This process of "arm swapping" was completed after applying the LGA loop building procedure to residues 39-41 in the shoulder regions. Finally, the LexA structures appear as both "elbows up" (N-terminal arm unbound) and "elbows down" (N-terminal arm bound to C-terminal domain), allowing us to model both conformations (Fig. S1). Thus, we created four models, two cis and two trans, each with an elbows up and an elbows down conformation (it is possible that heterogeneous conformations also occur with one elbow up and one elbow down as in the 1jhh LexA structure). For all the cis and trans models of the UmuD homodimer, the conformations of side chains from residues either not present in 1ay9_A or that presented a steric clash after building the dimeric structures were modeled using the side chain placement program SCWRL (26).

Proteins, Strains, and Plasmids—A plasmid expressing UmuD-3A was constructed in pSG5 using mutagenic primers and the QuikChange kit (Stratagene) (27). Wild-type UmuD and UmuD-3A were purified according to the published procedure (27). The plasmid expressing His-HMK-beta was a gift from Prof. M. O'Donnell (Rockefeller University), and beta was purified according to the published procedure (28). The strains and plasmids used in this study are listed in Table 1. Plasmid pSJS9 was a gift from Prof. Charles McHenry (University of Colorado). Site-directed mutagenesis was performed using the QuikChange kit (Stratagene). Primer sequences are published in the supplementary materials.


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  		TABLE 1
Strains and plasmids

 
Mutagenesis and Survival Assays—SOS mutagenesis assays were performed according to the published method (27). Briefly, cultures of GW8017 harboring various umuDC-expressing plasmids growing exponentially in LB were washed with 0.85% saline, exposed to 25 J/m2 UV light from a germicidal lamp (General Electric), and then plated on M9 minimal plates with trace arginine (1 µg/ml). Colony-forming units were scored after 48 h of growth at 42 °C. Survival was determined by plating on M9 minimal plates with 40 µg/ml arginine. Non-UV irradiated cultures were treated identically to assess the spontaneous mutation frequency. The data represent the average of at least three determinations.

UV survival curves were obtained after treating cells suspended in 0.85% saline in a Petri dish with the indicated doses of 254-nm light. Each sample was serially diluted and the dilutions were plated on M9 minimal media plates supplemented with 1% casamino acids, 0.005% tryptophan, and 1.5% agar. Plates were incubated overnight at 42 °C.

Quantitative Transformation Assays—Transformation assays were performed essentially as described (27). Plasmids (0.1 µg) were added to 25-50 µl of competent AB1157 cells and incubated on ice for 10 min. After a 5-min heat shock at 37 °C, and a further 10-min incubation on ice, transformation mixtures were allowed to recover in 750 µl of LB at 37 °C for 1.5 h with gentle shaking. Equal volumes were plated on LB plates containing the appropriate antibiotics for incubation under different temperatures as indicated in the figure legends.

Immunoblots—To determine UmuD expression levels, cells were harvested from exponentially growing cultures in LB, lysed by boiling for 15 min, and loaded on 4-20% SDS-polyacrylamide gradient gels (Cambrex). Electrophoresed proteins were transferred to polyvinylidene difluoride membrane (Millipore) in 10 mM CAPS, pH 8, 10% methanol. After blocking, membranes were probed with anti-UmuD/D', and antibody interactions were detected with SuperSignal substrate (Pierce). Antibodies to UmuD/D' were raised against purified UmuD in rabbits (Immunodynamics, Inc., La Jolla, CA). For UV-induced expression and cleavage experiments, an aliquot of about 2.5 x 1010 cells from an exponentially growing culture at A600 = 0.2-0.3 was harvested, washed in 0.85% saline, and UV-irradiated at 25 J/m2. Irradiated cells were then transferred to LB and grown at 37 °C for the times indicated in the figure legends.

UmuD in Vitro Cleavage Assay—RecA/ssDNA nucleoprotein filament-facilitated UmuD cleavage was assayed (27, 29, 30) in LG buffer for 30 min. Reactions were quenched by addition of SDS-PAGE buffer to 1x, and products were analyzed on 4-12% gradient polyacrylamide gels. Alkaline cleavage of UmuD was carried out (27, 31) in 100 mM glycine, pH 10, 10 mM CaCl2, 50 mM NaCl, 10 mM dithiothreitol, and 0.25 µg/ml bovine serum albumin for 48 h at 37 °C. Reaction products were analyzed by 14% polyacrylamide gel electrophoresis. The extent of UmuD cleavage was quantified using the Line Profile feature of the National Instruments Vision Assistant.

Cross-linking and Chemical Modification—Cross-linking was performed essentially as described (32) with bis-maleimidohexane (BMH, Pierce). Reactions were incubated at room temperature for the times indicated. For chemical modification with 5,5'-dithiobis(2-nitrobenzoate) (DTNB) (33), DTNB was dissolved at 2 mM final concentration in 50 mM HEPES, pH 7.5. Reactions were performed with 10-20 µM DTNB and 10-20 µM UmuD proteins in 50 mM HEPES, pH 7.5. The concentration of accessible thiols was calculated with an extinction coefficient of 13,600 cm-1 M-1 at 412 nm. Several trials were performed, and representative data are shown.


Figure 1
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  		FIGURE 1.
Homology model of the UmuD2 dimer (trans, arms down). Also see supplemental Fig. S1. One monomer is in blue; the other is in red. The residues mutated in UmuD-3A (Thr14 (orange); Leu17 and Phe18 (purple)) are shown in space-filling rendering. The single Cys residue, Cys24, is shown in yellow. Ser60 is shown in green.

 
Fluorescence Determination of Binding Constants—Binding constants between UmuD and beta were determined essentially as described, with a PT1 QM-20000-4SE spectrofluorimeter (Lawrenceville, NJ) (27). The beta clamp has a single Trp (residue 122), whereas UmuD has none. The beta2 concentration was constant at 2.5 µM. Emission from UmuD or UmuD-3A without beta2 was subtracted from emission of the complex, and the center of the spectral mass was calculated for each [UmuD]. Excitation was at 278 nm, and emission was monitored from 300 to 400 nm. Excitation and emission path polarizers were oriented perpendicularly. The data represent the average of at least three independent experiments ± 1 S.D.


Figure 2
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  		FIGURE 2.
The beta-binding motif variants do not eliminate UV-induced mutagenesis. A, the putative beta-binding motif in UmuD, with the variants indicated. B, induced mutation frequency of the indicated UmuD mutations in pGY9739 umuDC plasmids in GW8017 ({Delta}umuDC).

 

   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Mutations in the "beta-Binding Motif" of UmuD Do Not Result in Complete Loss of Induced Mutagenesis—We used alanine-scanning mutagenesis to make single alanine mutations in the putative beta-binding motif in UmuD (Figs. 1 and 2) and investigated the consequences of these variants on known phenotypes of UmuD. These plasmid-borne variants were assayed for their ability to complement a {Delta}umuDC null strain for UV-induced mutagenesis. In addition to single alanine variants of UmuD, we constructed one variant with alanines at the first and last two positions in the motif (UmuD-3A, Fig. 2), the positions most conserved among all beta-binding motifs (21). In the case of UmuC, the analogous mutation in its beta-binding motif results in a complete loss of UV-induced mutagenesis (11, 12). No single mutation or set of multiple mutations in this motif in UmuD failed completely to complement a {Delta}umuDC strain. However, plasmids expressing either the F18A UmuD variant, which is located at the top of the arm over the C-terminal globular domain, or the T14A/L17A/F18A ("UmuD-3A") variant resulted in substantial decreases in induced mutagenesis, down to about 15-20% of the wild-type (Fig. 2). Curiously, this decrease in mutagenesis of cells harboring these variants was not accompanied by a corresponding decrease in survival after UV irradiation (Fig. 5); yet, typically, increased mutagenesis due to translesion synthesis by UmuD'2C is associated with increased survival after treatment with UV.

Cleavage of the N-terminal 24 amino acids from the arm of UmuD to yield UmuD' is required to activate UmuC for its role in translesion synthesis (1). Because the UmuD arm harboring these mutations would be removed upon cleavage, we reasoned that the defect in induced mutagenesis of strains expressing the F18A and T14A/L17A/F18A (UmuD-3A) variant proteins might be because of defects in cleavage. Given their positions in the N-terminal arm (Fig. 1), it might be expected that these residues would play a role in properly positioning the arm in the active site for cleavage. We tested whether these mutations in the N-terminal arm of UmuD interfered with cleavage after UV exposure. The F15A and L17A mutants showed a slight decrease in cleavage and an ~1.5-fold decrease in induced mutagenesis compared with the wild-type (Figs. 2 and 3 and data not shown). The two UmuD variants (F18A and UmuD-3A) that showed essentially no cleavage up to 3 h after UV exposure (Fig. 3), or even after 14 h (data not shown), resulted in a substantial but not complete reduction in induced mutagenesis (about 15-20% of wild-type). This is in contrast to non-cleavable UmuD active site variants that have been assayed previously, which showed essentially complete loss of induced mutagenesis (the limit of detection of this assay is about 1000-fold, or 0.1% of wild-type) (34, 35). Thus, there are two groups of non-cleavable UmuD variants, one that renders cells partially mutable and the other that renders cells essentially non-mutable.

The wild-type and UmuD-3A variant proteins were purified to assess their efficiency in in vitro cleavage facilitated by the RecA/ssDNA nucleoprotein filament. Under these conditions, UmuD2 is cleaved efficiently to form UmuD'2, whereas UmuD-3A2 exhibits little detectable cleavage (Fig. 3). Here again, the lack of cleavage is similar to that exhibited by the active site mutant of UmuD2, UmuD-S60A2 (34). We note that there is a lower band present in some of the samples incubated without the RecA/ssDNA nucleoprotein filament. This lower band is often observed in preparations of UmuD, and even in some preparations of UmuD-S60A. However, in the case of UmuD-3A, the intensity of the lower band does not increase after incubation with the RecA/ssDNA nucleoprotein filament.

UmuD forms exchangeable dimers (34), so wild-type UmuD2 was combined with UmuD-3A2, and cleavage was observed (Fig. 3). Because UmuD-3A cannot cleave its own arm, the observed cleavage is likely because of the active site catalytic dyad of UmuD-3A acting on the arm of the wild-type partner, although the reverse is also possible. In this experiment it is also possible that the cleavage observed is due entirely to a small population of wild-type UmuD2 homodimers. To eliminate this possibility, UmuD-3A2 was incubated with the active site variant UmuD-S60A2, and some cleavage was still detected (Fig. 3). This slight cleavage must be due to the active site residues of UmuD-3A cleaving the arm of UmuD-S60A, which suggests that the active site of UmuD-3A is proficient for cleavage and that the cleavage defect is isolated to its arm. The mutations in UmuD-3A at the top of the arm may disrupt folding of the arm over the globular domain or may interfere with specific protein-protein contacts required to facilitate cleavage (Fig. 1).


Figure 3
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  		FIGURE 3.
UmuD variants are defective in cleavage. A, immunoblot showing the stable production of UmuD variant proteins. B, immunoblot showing in vivo cleavage of UmuD variant proteins. Time points for each sample are 0 (before UV irradiation) and 1, 2, and 3 h after UV irradiation. C, UmuD cleavage in vitro. The plus (+) sign indicates the presence of the RecA/ssDNA nucleoprotein filament. The last two lanes show alkaline cleavage of UmuD. The percentage of the cleaved product was determined as the proportion of total density of each lane.

 
To ensure that the cleavage defect was not because of defective interactions of UmuD-3A2 with the RecA/ssDNA nucleoprotein filament, we also carried out cleavage under alkaline conditions in the absence of the RecA/ssDNA nucleoprotein filament. The RecA/ssDNA nucleo-protein filament serves to facilitate deprotonation of Ser60 by a neutral Lys97 (4, 25). In the absence of the RecA/ssDNA nucleoprotein filament, the activation of Ser60 as a nucleophile can be accomplished under alkaline conditions. Under these conditions, UmuD2 cleavage is inefficient but can be detected (31). We found that cleavage of UmuD-3A2 was substantially decreased compared with that of the wild-type (Fig. 3). This suggests that the cleavage defect of UmuD-3A2 is due to a defect intrinsic to the UmuD-3A2 variant rather than deficient interactions with the RecA/ssDNA filament.

UmuD-3A Fails to Exhibit the Cold-sensitive Phenotype—Strains with elevated levels of the umuDC gene products exhibit a cold-sensitive phenotype that correlates with a DNA damage checkpoint (2, 3). Cells harboring plasmids overexpressing the cleavable umuD variants T14A and F15A (+wild-type umuC) were also cold-sensitive, whereas those overexpressing the L17A variant displayed an intermediate phenotype. The T14A and F15A variants behave similarly to wild-type in terms of their ability to exert the cold-sensitive phenotype, to be cleaved to UmuD', and to act in UV-induced mutagenesis. The cold-sensitive phenotype is substantially enhanced in cells overexpressing the non-cleavable variant UmuD-S60A (Fig. 4) (36). Thus, we were surprised to find that strains harboring plasmids expressing the noncleavable umuD arm variants (UmuD-F18A and UmuD-3A) failed to display this coldsensitive phenotype (Figs. 1 and 4).


Figure 4
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  		FIGURE 4.
UmuD variants result in loss of the cold-sensitive phenotype. The ratio of colony-forming units (cfu) of AB1157 per µg of transformed plasmid DNA when grown at 42 versus 30 °C is plotted for each UmuD construct.

 
We hypothesized that the loss of the cold-sensitive phenotype was due to the specific arm mutations of UmuD, regardless of their cleavage defect. To test this, we combined in single constructs either the UmuD-F18A or UmuD-3A arm mutations with the S60A mutation that renders UmuD catalytically inactive. Even though they are not cleavable, the arm mutations F18A and UmuD-3A alleviated the extreme cold sensitivity exhibited by strains with elevated levels of UmuD-S60A (Fig. 4). Although strains harboring plasmids overexpressing the UmuD F18A/S60A double mutant display a cold-sensitive phenotype that is intermediate between that of cells with each corresponding single mutant, the cold-sensitive phenotype of cells overexpressing UmuD-S60A is suppressed by 2 orders of magnitude by the presence of only a single mutation in the N-terminal arm, F18A. These arm variants must disrupt a specific molecular interaction necessary to cause the cold-sensitive phenotype that is independent of whether they can be cleaved.

Simultaneously elevated levels of the umuD, umuC, and dnaN (which codes for the beta clamp) gene products cause a lethal phenotype, which has been interpreted as an exaggeration of the cold-sensitive phenotype (37). A strain harboring a plasmid expressing UmuD-3A and UmuC, when combined with high levels of the beta clamp, fails to exhibit the synthetic lethal phenotype (Table 2). This suggests that a critical aspect of this complex formation with beta is disrupted in the UmuD-3A variant.


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  		TABLE 2
Loss of synthetic lethality due to mutations in UmuD beta-binding motif

 


Figure 5
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  		FIGURE 5.
UV survival of UmuD variants. Assays were performed with pGY9739 plasmids and derivatives in GW8017 ({Delta}umuDC): pGY9738 (umuD'C,{diamondsuit}); pGY9739-F18A (umuDC F18A, bullet); pGY9739-D3A S60A (umuDC D3A/S60A, +); pGY9739-F18A/S60A (umuDC F18A/S60A, -); pGY9739-D3A (umuDC D3A, *); pGY9739 (umuDC, {blacksquare}); pGB2 (empty vector, {blacktriangleup}); and pGY9739-S60A (umuDC S60A, x).

 


Figure 6
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  		FIGURE 6.
UmuD-3A arms are more easily cross-linked with BMH and more accessible to chemical modification than wild-type. A, cross-linking the UmuD arms with BMH. Each UmuD variant that was cross-linked is indicated under the lanes. The time points were 0, 15, and 30 min after addition of BMH. The first lane shows molecular mass standards, indicated in kDa. Proteins were visualized by Coomassie staining. B, DTNB titration of free thiol in UmuD. The complete reaction without protein was used as a blank, protein was added and absorbance was recorded at 412 nm for 15 min. UmuD2 ({blacksquare}) and UmuD-3A2 ({diamondsuit}) were present at 10 µM with 20 µM DTNB.

 
Sensitivity to UV Exposure—Given the cleavage defect of the UmuD-3A and F18A N-terminal arm variants, we decided to look more closely at the curious lack of a correlation between UV mutability and survival after exposure to UV that we had noted earlier for cells expressing the F18A and UmuD-3A variants. It is known that E. coli strains harboring a noncleavable UmuD variant are hypersensitive to killing by UV light and are non-mutable (2, 34) (Fig. 5). To determine whether this is also true of the non-cleavable UmuD F18A and UmuD-3A variants, strains harboring plasmid-borne umuD variants were assayed for their resistance to UV light. Strains with plasmids expressing either of the non-cleavable variants F18A or UmuD-3A showed a similar level of resistance to UV light as those expressing wild-type UmuD or a synthetic construct of UmuD' (Fig. 5). We suspected that the resistance to killing by UV of the F18A and UmuD-3A variants was because of a specific feature of the arm mutants, unrelated to their cleavage defect. Cells with plasmids expressing the non-cleavable arm variants constructed in the context of the UmuD-S60A active site variant were assayed for their resistance to UV light. Strains with plasmids expressing either the F18A or UmuD-3A arm variants combined with UmuD-S60A exhibited a striking resistance to UV light that was similar to strains with wild-type UmuD (Fig. 5). This suggests that alterations in the N-terminal arm of UmuD are able to suppress the extreme UV-sensitive phenotype of non-cleavable UmuD-S60A, even though they are also non-cleavable. In light of the dramatic changes in the phenotypes of cells expressing the UmuD-3A variant compared with those with previously characterized non-cleavable variants of UmuD (34), we investigated the conformational consequences of the UmuD-3A variant compared with the wild-type.

Chemical Cross-linking and Modification of UmuD Homodimers—We hypothesized that these non-cleavable UmuD variants are able to confer resistance to UV light, as well as to suppress the cold-sensitive phenotype, by mimicking the conformation of UmuD'. To examine this possibility, we analyzed the conformation of the N-terminal arm of the UmuD-3A variant compared with that of the wild-type UmuD. UmuD, which possesses a C2 axis of symmetry, has a single Cys residue, Cys24, at the cleavage site in the N-terminal arm. To determine whether UmuD-3A is a UmuD' mimic with respect to the position of its arms, cross-linking was performed with the thiol-specific homobifunctional 16-Å cross-linker BMH. Our model allows us to put a lower limit of 20 Å on the distance between these two Cys thiols. This lower limit represents an implausible path for the cross-linker, because it is the direct distance between the two Cys thiols (Fig. 1). Thus, cross-linking should only be detected when the arms are "up," i.e. not bound to the C-terminal globular domain of UmuD. UmuD-3A was more readily crosslinked by BMH than either wild-type UmuD or UmuD-S60A (Fig. 6), suggesting that the arms of UmuD-3A are less likely to be bound to the globular domain of UmuD. Therefore they are more often close enough to be cross-linked.

One of the models of UmuD that we have proposed ("trans, elbows down," see Fig. 1 and supplemental Fig. S1) predicts that the thiol group of the single Cys24 residue is partially buried under the peptide backbone of the N-terminal arm. However, if the UmuD arms are in an "up," or more flexible, conformation, then the Cys should be more accessible to a thiol-specific reagent. We performed a titration of the Cys residue at the Cys24-Gly25 cleavage site with DTNB. The thiol moiety of UmuD-3A was more reactive to DTNB and therefore slightly more accessible than that of the wild-type UmuD (Fig. 6). We determined that there is 1.0 reactive Cys residue per wild-type UmuD2 and 1.2 reactive Cys residues per UmuD-3A2, supporting the idea that in the UmuD-3A variant, the N-terminal arms undergo a shift in equilibrium to a less bound, arms-up state.

Determination of Kd of UmuD and the beta Clamp—To quantify the binding of UmuD and the UmuD-3A variant to the beta clamp, we determined the Kd for this interaction. Surprisingly, we found that although the Kd is similar for beta binding to either wild-type UmuD (5.5 ± 0.8 µM) or UmuD-3A (6.1 ± 0.5 µM), the mode of binding is different for each protein. Namely, the fluorescence emission from the tryptophan in beta shifts to a longer wavelength upon binding to UmuD, whereas the shift is to a shorter wavelength in the presence of UmuD-3A (Fig. 7). Tryptophan fluorescence emission peaks at a longer wavelength in a polar environment and at a shorter wavelength in a hydrophobic one, indicating that the partially exposed tryptophan in beta (Fig. 7B) becomes more solvent exposed upon binding to wild-type UmuD and buried upon binding to UmuD-3A (38). Accordingly, unlike canonical beta-binding motifs (11, 12, 39, 40), this motif in UmuD is not responsible for the strength of the interaction with beta, but rather for a qualitatively different mode of binding.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Although this work was initiated to determine the role of the putative beta-binding motif (14TFPLF18) in UmuD function, we found that alterations in the motif do not prevent binding to the beta clamp, unlike corresponding mutations in UmuC, DinB, and the pol III {alpha} subunit (11, 12, 21, 39, 40). Instead, we show here that the UmuD-3A variant alters the N-terminal arm conformation in a way that dramatically changes UmuD activity, and seems to exhibit properties of UmuD', particularly with respect to resistance to killing by exposure to UV light and lack of the cold-sensitive phenotype when overexpressed together with UmuC. The UmuD-3A variant shows no defect in survival but decreases UV mutagenesis. This suggests that this variant may allow selective bypass of T:T cyclobutane pyrimidine dimers but not (6-4)-photoproducts, because lethality is associated with T:T cyclobutane pyrimidine dimers and mutagenesis with (6-4)-photoproducts (1).


Figure 7
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  		FIGURE 7.
UmuD binding as observed by beta tryptophan fluorescence. A, UmuD (filled circles) and UmuD-3A (open circles) bind with similar affinity yet cause opposite shifts in the Trp fluorescence emission wavelength. The Kd of UmuD2 is 5.5 ± 0.8µM, whereas that of UmuD-3A2 is 6.1 ± 0.5 µM. Representative fluorescence emission curves, as well as the full data set of the Centers of Spectral Mass as a function of UmuD concentration, are presented in the supplementary materials. B, structure of the beta clamp (Protein Data Bank code 2POL [PDB] ) showing the known sites of interaction of UmuD based on cross-linking experiments (red, residues 2, 74, 170, and 363) (20) versus the site of interaction of the canonical beta-binding motif (green, residues 247 and 360) and the second site of interaction observed in the co-crystal structure with DinB (green, labeled second site; residues 93 and 98) (44, 45). The single tryptophan, Trp122, is also indicated in yellow. Arrows indicate the dimer interface. The domains are labeled I, II, and III. This image was prepared with VMD (50).

 
How could an uncleaved UmuD mimic the cleaved form, UmuD'? Current evidence suggests that the N-terminal arms of UmuD are usually bound to the C-terminal globular domain of the protein, i.e. it is usually in the elbows down conformation (7). Even when the arms are covalently bound to the globular domain, UmuD can be cross-linked to the beta clamp with almost no decrease in efficiency (10). When UmuD is cleaved to UmuD', the remainder of the N-terminal arm (residues 25-39) is able to move relatively freely (5, 24). The UmuD-3A variant seems to have enough flexibility in its arms that it is at least a partial mimic of UmuD'. Although residues 14-18 of UmuD are predicted from our model to be only partially buried (Fig. 1), the UmuD-3A variant may disrupt optimal packing of the arm against the globular domain. The mutations in UmuD-3A are at the point of the N-terminal arm that begins a downward turn over the C-terminal globular domain, and disruption of this turn is consistent with the more extended UmuD' arm structure. In addition, the cleavage of UmuD exposes a different surface of UmuD' for protein-protein interactions. Thus, the cleavage reaction serves the dual function of removing a portion of the N-terminal arms and presenting a dramatically different surface of the protein for interactions.

We were able to create models of the UmuD homodimer in both the elbows up and the elbows down conformation because both conformations are observed in the LexA structures (25). The LexA structure is in a cis conformation (non-domain swapped) with respect to the positioning of the arms (25). We have noticed, however, that in the UmuD'2 structures the truncated arms point in the trans direction, suggesting that perhaps the trans conformation is actually preferred for UmuD (5, 6). It has been shown that UmuD can undergo cleavage in trans (Fig. 3) (5, 41). The model of UmuD most consistent with the available biochemical evidence is one in which the arms are in the trans conformation (7).

In constructing the UmuD-3A variant, we have made a version of UmuD that binds to the beta clamp with a similar affinity as the wild-type, but with a subtle change in the specific interaction as evidenced by the strikingly different tryptophan fluorescence emission spectra. This change would not have been detected by many of the techniques commonly used to detect protein-protein interactions, such as co-immunoprecipitation or two-hybrid analysis. Recent evidence suggests that the domains of the sliding clamps are rigid bodies joined by flexible linker regions (42). The single tryptophan of beta is on a long flexible loop between rigid Domains I and II (Fig. 7) (43), so UmuD binding at a distal site to the tryptophan could cause a slight conformational rearrangement of the domains that alters the environment of the tryptophan in the loop. The distinct fluorescence spectra are indicative of at least a slight change in the conformation of the beta clamp when bound to either wild-type UmuD or UmuD-3A. UmuD-3A exhibits similar biological functions to those of UmuD', yet UmuD-3A retains both the arm length and the strength of the interaction with beta of wild-type UmuD. This suggests that the change in conformation of the beta clamp, along with the relative conformational freedom of the UmuD-3A N-terminal arms, may be partially responsible for the UmuD'-like phenotype of UmuD-3A. UmuD and DinB/pol IV bind to beta at overlapping sites (Fig. 7) (20, 44). One of these sites is the hydrophobic channel between beta Domains II and III where all known beta-binding motifs interact (44-46). Although UmuD possesses a similar motif to a canonical beta-binding motif that modulates its interaction with the beta clamp, and UmuD has been shown to bind to the same site on beta as other beta-binding motifs, it does so in a way that is distinct from other proteins that bind beta via their beta-binding motifs (46).

Processivity clamps play a critical role in controlling traffic at the replication fork and in cell cycle checkpoints. Polymerase binding to the beta clamp regulates access to the primer terminus by replicative or translesion synthesis DNA polymerases (13, 14). Moreover, it has been shown that UmuD interacts more strongly with beta than UmuD' does (9). The interaction of UmuD with the beta clamp seems to be important for facilitating a DNA damage checkpoint in E. coli (2, 3), and the cleavage of UmuD to UmuD' may attenuate this checkpoint function (10). The UmuD-3A variant seems to bypass this switch, yet still binds the beta clamp. In eukaryotes, covalent modification of proliferating cell nuclear antigen with monoubiquitin or small ubiquitin-like modifier (SUMO) determines whether the cell utilizes DNA repair or potentially mutagenic translesion synthesis (17). Thus, access to sliding clamps is universally important for control and regulation of proteins acting at the replication fork.


   FOOTNOTES
 
* This work was supported in part by National Institutes of Health NCI Grant CA21615, National Institutes of Health NIEHS Center Grant P30ES02109 from the MIT Center for Environmental Health Sciences, an American Cancer Society Professorship (to G. C. W.), and a postdoctoral fellowship from the Damon Runyon Cancer Research Foundation (to P. J. B.). Some of this work was performed under the auspices of the United States Department of Energy by the University of California, Lawrence Liver-more National Laboratory Contract W-7405-Eng-48. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

Formula The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1 and S2 and Table S1. Back

1 Supported by a Cleo and Paul Schimmel Fellowship. Back

2 To whom correspondence should be addressed: 68-633, 77 Massachusetts Ave., Cambridge, MA 02139. Tel.: 617-253-6716; Fax: 617-253-2643; E-mail: gwalker{at}mit.edu.

3 The abbreviations used are: ssDNA, single-stranded DNA; CAPS, 3-(cyclohexylamino)propanesulfonic acid; BMH, bis-maleimidohexane; DTNB, 5,5'-dithiobis(2-nitrobenzoate). Back


   ACKNOWLEDGMENTS
 
We thank Prof. Robert Sauer (MIT) and members of his laboratory for use of the fluorimeter and for technical advice. We also thank Prof. Charles McHenry for pSJS9 and helpful discussions. We thank Prof. Mike O'Donnell (Rockefeller University) for the plasmid expressing His-HMK-beta, Daniel Jarosz for assistance in preparing Fig. 7, and Michael Simon and members of the Walker laboratory for careful reading of the manuscript.



   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Friedberg, E. C., Walker, G. C., Siede, W., Wood, R. D., Schultz, R. A., and Ellenberger, T. (2005) DNA Repair and Mutagenesis, Second Ed., ASM Press, Washington, D.C
  2. Opperman, T., Murli, S., Smith, B. T., and Walker, G. C. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 9218-9223[Abstract/Free Full Text]
  3. Sutton, M. D., and Walker, G. C. (2001) J. Bacteriol. 183, 1215-1224[Abstract/Free Full Text]
  4. Ferentz, A. E., Walker, G. C., and Wagner, G. (2001) EMBO J. 20, 4287-4298[CrossRef][Medline] [Order article via Infotrieve]
  5. Ferentz, A. E., Opperman, T., Walker, G. C., and Wagner, G. (1997) Nat. Struct. Biol. 4, 979-983[CrossRef][Medline] [Order article via Infotrieve]
  6. Peat, T. S., Frank, E. G., McDonald, J. P., Levine, A. S., Woodgate, R., and Hendrickson, W. A. (1996) Nature 380, 727-730[CrossRef][Medline] [Order article via Infotrieve]
  7. Sutton, M. D., Guzzo, A., Narumi, I., Costanzo, M., Altenbach, C., Ferentz, A. E., Hubbell, W. L., and Walker, G. C. (2002) DNA Repair 1, 77-93[CrossRef][Medline] [Order article via Infotrieve]
  8. Shen, X., Woodgate, R., and Goodman, M. F. (2003) J. Biol. Chem. 278, 52546-52550[Abstract/Free Full Text]
  9. Sutton, M. D., Opperman, T., and Walker, G. C. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 12373-12378[Abstract/Free Full Text]
  10. Sutton, M. D., Narumi, I., and Walker, G. C. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 5307-5312[Abstract/Free Full Text]
  11. Becherel, O. J., Fuchs, R. P., and Wagner, J. (2002) DNA Repair 1, 703-708[CrossRef][Medline] [Order article via Infotrieve]
  12. Beuning, P. J., Barsky, D., Sawicka, D., and Walker, G. C. (2006) Mol. Microbiol. 59, 460-474[CrossRef][Medline] [Order article via Infotrieve]
  13. Fujii, S., and Fuchs, R. P. (2004) EMBO J. 23, 4342-4352[CrossRef][Medline] [Order article via Infotrieve]
  14. Indiani, C., McInerney, P., Georgescu, R., Goodman, M. F., and O'Donnell, M. (2005) Mol. Cell 19, 805-815[CrossRef][Medline] [Order article via Infotrieve]
  15. Yang, J., Zhuang, Z., Roccasecca, R. M., Trakselis, M. A., and Benkovic, S. J. (2004) Proc. Natl. Acad. Sci. U. S. A. 101, 8289-8294[Abstract/Free Full Text]
  16. Kannouche, P. L., Wing, J., and Lehmann, A. R. (2004) Mol. Cell 14, 491-500[CrossRef][Medline] [Order article via Infotrieve]
  17. Gill, G. (2004) Genes Dev. 18, 2046-2059[Abstract/Free Full Text]
  18. Vivona, J. B., and Kelman, Z. (2003) FEBS Lett. 546, 167-172[CrossRef][Medline] [Order article via Infotrieve]
  19. Warbrick, E. (2000) Bioessays 22, 997-1006[CrossRef][Medline] [Order article via Infotrieve]
  20. Duzen, J. M., Walker, G. C., and Sutton, M. D. (2004) DNA Repair 3, 301-312[CrossRef][Medline] [Order article via Infotrieve]
  21. Dalrymple, B. P., Kongsuwan, K., Wijffels, G., Dixon, N. E., and Jennings, P. A. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 11627-11632[Abstract/Free Full Text]
  22. Zemla, A. (2003) Nucleic Acids Res. 31, 3370-3374[Abstract/Free Full Text]
  23. Zemla, A., C., E. Z., Slezak, T., Kuczmarski, T., Rama, D., Torres, C., Sawicka, D., and Barsky, D. (2005) Nucleic Acids Res. 33, W111-W115[Abstract/Free Full Text]
  24. Peat, T. S., Frank, E. G., McDonald, J. P., Levine, A. S., Woodgate, R., and Hendrickson, W. A. (1996) Structure 4, 1401-1412[Medline] [Order article via Infotrieve]
  25. Luo, Y., Pfuetzner, R. A., Mosimann, S., Paetzel, M., Frey, E. A., Cherney, M., Kim, B., Little, J. W., and Strynadka, N. C. J. (2001) Cell 106, 585-594[CrossRef][Medline] [Order article via Infotrieve]
  26. Bower, M., Cohen, F. E., and Dunbrack, R. L., Jr. (1997) J. Mol. Biol. 267, 1268-1282[CrossRef][Medline] [Order article via Infotrieve]
  27. Beuning, P. J., Simon, S. M., Godoy, V. G., Jarosz, D. F., and Walker, G. C. (2006) Methods Enzymol. 408, 318-340[Medline] [Order article via Infotrieve]
  28. Turner, J., Hingorani, M. M., Kelman, Z., and O'Donnell, M. (1999) EMBO J. 18, 771-783[CrossRef][Medline] [Order article via Infotrieve]
  29. Guzzo, A., Lee, M. H., Oda, K., and Walker, G. C. (1996) J. Bacteriol. 178, 7295-7303[Abstract/Free Full Text]
  30. Lee, M. H., Guzzo, A., and Walker, G. C. (1996) J. Bacteriol. 178, 7304-7307[Abstract/Free Full Text]
  31. Kulaeva, O. I., Wootton, J. C., Levine, A. S., and Woodgate, R. (1995) J. Bacteriol. 177, 2737-2743[Abstract/Free Full Text]
  32. Lee, M. H., Ohta, T., and Walker, G. C. (1994) J. Bacteriol. 176, 4825-4837[Abstract/Free Full Text]
  33. Wright, S. K., and Viola, R. E. (1998) Anal. Biochem. 265, 8-14[CrossRef][Medline] [Order article via Infotrieve]
  34. Battista, J. R., Ohta, T., Nohmi, T., Sun, W., and Walker, G. C. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 7190-7194[Abstract/Free Full Text]
  35. Nohmi, T., Battista, J. R., Dodson, L. A., and Walker, G. C. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 1816-1820[Abstract/Free Full Text]
  36. Opperman, T., Murli, S., and Walker, G. C. (1996) J. Bacteriol. 178, 4400-4411[Abstract/Free Full Text]
  37. Sutton, M. D., Farrow, M. F., Burton, B. M., and Walker, G. C. (2001) J. Bacteriol. 183, 2897-2909[Abstract/Free Full Text]
  38. Harris, D. A. (1996) Light Spectroscopy, BIOS Scientific Publishers, Ltd., Oxford, UK
  39. Dohrmann, P. R., and McHenry, C. S. (2005) J. Mol. Biol. 350, 228-239[CrossRef][Medline] [Order article via Infotrieve]
  40. Wagner, J., Fujii, S., Gruz, P., Nohmi, T., and Fuchs, R. P. (2000) EMBO Rep. 1, 484-488[Medline] [Order article via Infotrieve]
  41. McDonald, J. P., Frank, E. G., Levine, A. S., and Woodgate, R. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 1478-1483[Abstract/Free Full Text]
  42. Kazmirski, S. L., Zhao, Y., Bowman, G. D., O'Donnell, M., and Kuriyan, J. (2005) Proc. Natl. Acad. Sci. U. S. A. 102, 13801-13806[Abstract/Free Full Text]
  43. Kong, X.-P., Onrust, R., O'Donnell, M., and Kuriyan, J. (1992) Cell 69, 425-437[CrossRef][Medline] [Order article via Infotrieve]
  44. Bunting, K. A., Roe, S. M., and Pearl, L. H. (2003) EMBO J. 22, 5883-5892[CrossRef][Medline] [Order article via Infotrieve]
  45. Jeruzalmi, D., Yurieva, O., Zhao, Y., Young, M., Stewart, J., Hingorani, M., M., and Kuriyan, J. (2001) Cell 106, 417-428[CrossRef][Medline] [Order article via Infotrieve]
  46. Wijffels, G., Dalrymple, B. P., Prosselkov, P., Kongsuwan, K., Epa, V. C., Lilley, P. Jergic, S., Buchardt, J., Brown, S. E., Alewood, P. F., Jennings, P. A., and Dixon, N. (2004) Biochemistry 43, 5661-5671[CrossRef][Medline] [Order article via Infotrieve]
  47. Sommer, S., Boudsocq, F., Devoret, R., and Bailone, A. (1998) Mol. Microbiol. 281-291
  48. Churchward, G., Belin, D., and Nagamine, Y. (1984) Gene (Amst.) 31, 165-171[CrossRef][Medline] [Order article via Infotrieve]
  49. Johanson, K. O., Haynes, T. E., and McHenry, C. S. (1986) J. Biol. Chem. 11460-11465
  50. Humphrey, W., Dalke, A., and Schulten, K. (1996) J. Mol. Graph. 14, 33-38[CrossRef][Medline] [Order article via Infotrieve]

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S. M. Simon, F. J. R. Sousa, R. Mohana-Borges, and G. C. Walker
Regulation of Escherichia coli SOS mutagenesis by dimeric intrinsically disordered umuD gene products
PNAS, January 29, 2008; 105(4): 1152 - 1157.
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