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J Biol Chem, Vol. 274, Issue 4, 2286-2290, January 22, 1999

Critical Nucleotides in the Upstream Region of the XylS-dependent TOL meta-Cleavage Pathway Operon Promoter as Deduced from Analysis of Mutants^*

M. Mar González-Pérez, Juan L. Ramos, María-Trinidad Gallegos, and Silvia Marqués

From the Department of Biochemistry and Molecular and Cellular Biology of Plants, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, Apartado de Correos 419, E-18008 Granada, Spain

	ABSTRACT

Top Abstract Introduction References

The Pm promoter, dependent on TOL plasmid XylS regulator, which is activated by benzoate effectors, drives transcription of the meta-cleavage pathway for the metabolism of alkylbenzoates. This promoter is unique in that in vivo transcription is mediated by RNA-polymerase with different sigma factors. In vivo footprinting analysis shows that XylS interacted with nucleotides in the 40 to 70 region. In vivo and in vitro methylation of Pm shows extensive methylation of T at position 42 in the bottom strand, suggesting that it represents a key distortion point that may favor XylS/RNA polymerase interactions. Methylation of T⁴² was highest in cells bearing XylS and in the presence of an effector. Gs in the 47 to 61 region appeared to be more protected in cells harboring XylS in the presence than in the absence of the effector. Almost 100 mutants in the Pm region between 41 and 78 were generated; transcriptional analysis of these mutants defined the XylS target as two direct repeats with the sequence TGCAN₆GGNCA. These motifs cover the 70 to 56 and the 49 to 35 regions. Single point mutations revealed that nucleotides located at 49 to 46 and at 59, 60, 62, and 70 are the most critical for appropriate XylS-Pm interactions.

	INTRODUCTION

Top Abstract Introduction References

The TOL plasmid pWW0 of Pseudomonas putida specifies a meta-cleavage pathway for the oxidative catabolism of benzoate and toluates. Genes encoding the TOL meta-cleavage pathway are grouped into a single operon, the expression of which is positively regulated at the level of transcription by the xylS gene product, which is activated by benzoate effectors (1-4). Stimulation of transcription from the Pm promoter requires a DNA sequence extending to about 80 bp¹ upstream of the transcription initiation point (5-7). On the basis of genetic data, two regions can be distinguished in the architecture of the Pm promoter: the XylS interaction region, which extends from about 40 to 80 bp (Fig. 1); and the downstream RNA polymerase recognition region, which exhibits atypical 35 and 10 DNA sequences (Fig. 1).

Transcription from the Pm promoter in the early exponential growth phase is mediated by RNA polymerase with ³², and later with ^S, although regardless of the growth phase, expression from Pm remains dependent on XylS, and the transcription initiation point is the same (8, 9).

Kessler et al. (6) proposed that the XylS binding region in Pm was organized as two homologous 15-bp tandemly imperfect directly repeated motifs (5'-TGCAAPuAAPyGGNTA-3'). The distal one with respect to the RNA polymerase region extends from 70 to 56, and the proximal one extends from 49 to 35 (Fig. 1). Gallegos et al. (5) also studied the organization of the XylS binding sites in the Pm promoter and suggested that sequences shorter than those proposed by Kessler et al. (6) might suffice for XylS activation of transcription. These workers found that promoters that had been deleted up to 60 could be activated by constitutive XylS mutants (but not by the wild-type regulator) and that extension of the deletion to 51 prevented transcription. Gallegos et al. (5) proposed that the XylS binding site is represented by the motif T(C/A)CAN₄TGCA, which appears twice in the promoter sequence, between 46 and 57 and between 67 and 78 (Fig. 1).

This study was designed to shed light on the nucleotides in the Pm promoter that are critical for XylS-dependent stimulation of transcription.

	MATERIALS AND METHODS

Bacterial Strains, Culture Medium, and Plasmids-- Escherichia coli MC4100 was grown at 30 °C in Luria-Bertani medium supplemented, when required, with 100 µg/ml ampicillin, 25 µg/ml kanamycin, or 50 µg/ml streptomycin.

The plasmids used in this study, and previously constructed were: pERD103, which is an IncQ plasmid encoding kanamycin resistance (7); pJLR100, which is a pEMBL9 derivative bearing the Pm promoter cloned between the EcoRI and HindIII sites (3); pMD1405, which carries a promoterless 'lacZ gene and encodes resistance to ampicillin; and pJLR107, which is a pMD1405 derivative bearing the Pm promoter in front of 'lacZ (3).

DNA Techniques-- DNA preparation, digestion with restriction enzymes, analysis by agarose gel electrophoresis, isolation of DNA fragments, ligations, transformations, and sequencing reactions were done according to standard procedures (10).

Methylation Experiments-- DNA was methylated in vitro with 2 mM dimethyl sulfate as described (10). For in vivo DNA metylation, E. coli cells bearing the Pm promoter in pMD1405 were exposed to 2 mM dimethyl sulfate for 1 min at 30 °C. Cells were processed as described (10). Plasmid DNA was extracted by using the Qiagen Kit (Quiagen, GmbH, Hilden, Germany).

The oligonucleotide 5'-CGTCTAAGAAACCATTATTATCAG-3' was complementary to the noncoding strand upstream from the Pm promoter region. The first C at the 5'-end was located 218 bp from the +1 of the transcription initiation point. The oligonucleotide 5'-GGGTCGGTGAACATCTCGCGCTTGC-3' was complementary to the coding strand downstream from the Pm promoter. The first G at the 5'-end was located 124 bp from the +1 of the start of the transcript.

For primer extension with Taq-DNA-polymerase, about 2 × 10⁵ cpm of the corresponding oligonucleotide end-labeled with ³²P was used. The extension products were separated by electrophoresis on urea-polyacrylamide sequencing gels.

Construction of Pm Mutant Promoters by Polymerase Chain Reaction-- The Pm mutant promoters were generated by overlap extension polymerase chain reaction mutagenesis as described (5). The internal oligonucleotide primers used for mutagenesis exhibited one or more mismatches with respect to the wild-type sequence. The external oligonucleotide was the so-called M13 reverse primer (5'-CAGGAAACAGCTATGACCATG-3') or a primer complementary to the  fragment of the lacZ gene (5'-GATGTGCTGCAAGGCGATTAAGTTA-3'). After DNA amplification, the resulting DNA was digested with EcoRI and HindIII, and the 401-bp EcoRI-HindIII Pm mutants were inserted between the EcoRI-HindIII sites of pMD1405 to yield plasmids pMARx (the x indicates the plasmid number from 5D to 296). All the mutant Pm promoters generated in this study were confirmed by DNA sequencing.

-Galactosidase Assays-- E. coli bearing the wild-type Pm::'lacZ or mutant Pm*::'lacZ fusions in pMD1405, plus pERD103, were grown overnight on Luria-Bertani medium containing the appropriate antibiotics. Cultures were diluted 100-fold in triplicate in the same medium supplemented or not with 1 mM 3MB. After 4 h of incubation, -galactosidase activity was determined in duplicate in permeabilized cells (5).

	RESULTS

In Vivo Methylation Assays Located the XylS Binding Region Adjacent to the RNA Polymerase Binding Site-- The reactivity of guanine residues in the Pm promoter toward dimethyl sulfate was assayed in vivo. DNA protection analysis was done in E. coli bearing only pJLR107 (Pm) or pJLR107 and pERD103 (XylS) and in the absence and in the presence of 3MB. Methylation was done when cells had reached the mid-logarithmic growth phase. As a control, Pm was also methylated in vitro. Representative results are shown in Fig. 2. In the bottom strand, a significant feature of the in vitro and in vivo methylation pattern is hypermethylation of the T located at 42. This indicates that the DNA was distorted at this point, probably due to the tracks of As in the 41 to 46 region in the top strand (Fig. 1). The methylation of T⁴² in vivo in cells without XylS was more pronounced than in vitro, indicating that this distortion may have been more pronounced in vivo. The methylation pattern of Pm in vivo in cells without XylS was very similar in the absence and in the presence of 3MB. However, the presence of the effector influenced the methylation pattern of Pm in cells bearing the XylS protein. When cells expressed the XylS protein, methylation of T⁴² was highest in the presence of the effector, whereas in the absence of the effector, T⁴² appeared to be more protected than in cells without XylS (Fig. 2). In contrast with this behavior was the observation that Gs in the 47 to 61 region appeared to be protected in the presence of the effector. However, G⁶⁸ appeared to be more methylated in the presence of effector (Fig. 2). This set of results suggested that the XylS protein is able to bind to Pm; however, the critical interactions could not be deduced from this assay.

View larger version (6K): [in this window] [in a new window]   Fig. 1.   Nucleotide sequence of the Pm promoter. The proposed +1 nucleotide is based on reverse transcriptase extension of primers that overlap the mRNA synthesized from this promoter (2, 31). The two arrows over the 70 to 35 region are nucleotides important for XylS recognition in Pm, as proposed by Kessler et al. (6); the two dashed arrows at the bottom represent the motif proposed by Gallegos et al. (5).

View larger version (80K): [in this window] [in a new window]   Fig. 2.   In vivo footprinting of XylS. E. coli MC4100 cells bearing only plasmid pJLR107 (Pm::'lacZ) or this plasmid plus pERD103 (xylS) were grown in the presence and in the absence of 3MB, and then exposed to dimethyl sulfate. The panel on the left shows the region of the bottom strand between 77 and 35, and the panel on the right shows the region of the top strand between 40 and +3 with respect to the main transcription initiation point. Lane V is the same DNA methylated in vitro. The G bases at 77, 68, 61, 56, 47, 39, 37, 24 and 3 are indicated, and the position corresponding to T at 42 is also shown.

In the top strand in Fig. 2, the 3 to 40 region showed a definite pattern of methylation. The Gs in the 3 to 28 region were more methylated in vitro than in vivo. In particular, G²³ and G³⁸ were clearly protected in vivo regardless of the presence of XylS and the presence of an effector.

Block Scanning Mutagenesis of the Putative XylS Binding Site at the Pm Promoter-- A characteristic of the members of the AraC/XylS family of transcriptional regulators is that they recognize short nucleotide motifs (4-6 bp) at their cognate promoters (11). For this reason, we decided to carry out initial block scanning mutagenesis assays of the Pm region between 41 and 78. This interval includes all nucleotides previously proposed as important in XylS for Pm recognition (5-7) and the region in which in vivo footprinting analyses revealed alterations in the methylation pattern. The interval excluded mutations in the putative 35 region of the Pm (Fig. 1), which, as shown in the in vivo footprinting analysis, may interfere with recognition of the promoter by the RNA polymerase. A series of 4-bp blocks to induce mutations were introduced randomly by polymerase chain reaction. These included the four TNCA motifs found in this region (46/49, 54/57, 67/70, and 75/78), the intervening sequences between two adjacent TNCA sequences (50/53, 58/61, 62/65, 63/66, and 71/74), a sequence between the TNCA sequence closer to the RNA polymerase binding site (46/49), and the 41 nucleotide (41/44). In all cases the mutant promoters (Pm^*) were fused to 'lacZ in pMD1405. -Galactosidase activity was determined in E. coli MC4100 (Pm^*::'lacZ, pERD103) grown in the absence and in the presence of 3MB. The basal level of expression from the Pm* promoters (50-100 Miller units) was similar to the basal level of expression determined from the Pm wild-type promoter. However, with regard to the induced levels of expression of the mutant promoters, three classes of Pm mutants were found (Table I): 1) mutant promoters that exhibited less than 20% of the wild-type activity; 2) mutant Pm promoters with a level of XylS-dependent expression between 65 and 25% of the expression measured with the wild-type promoter; and 3) mutant Pm promoters that conserved wild-type or near wild-type XylS-dependent inducible -galactosidase activity, i.e. 80% of wild-type levels.

The mutations that resulted in the largest reduction in transcription, i.e. a decrease equal to or greater than 80% of the wild-type activity, were any random substitution of the TGCA sequence between 46/49 (Table I). This suggests that these nucleotides are critical for XylS-dependent transcription activation of Pm. The substitution by random sequences of the AAAAA sequence located at 41/45, the TACA sequence between 54/57, the CGGA sequence between 58/61, and the TGCA sequence between 67/70 resulted in a significant decrease in XylS-dependent transcription activation of Pm. The activity of most of the mutant Pm promoters at these locations ranged from approximately 25 to about 65% of the activity of the wild-type promoter (Table I), although certain substitutions had little effect. These results suggest that these sets of bases are less critical than those at the 46/49 region; however, they may play a direct role in the recognition of the Pm DNA sequences by XylS, or they may contribute to the overall affinity for Pm. We cannot rule out other effects.

The third group of mutations, i.e. those that had no effect (or little effect) on transcription from Pm, were found to correspond to the locations of 50/53, 62/65, 63/66, 71/74, and 75/78.

We also investigated whether the combination of different blocks of mutations had a synergistic effect on XylS-dependent transcription activation from the mutant promoters. The combination of a block of mutations in 46/49 (TGCAAGGA) with a block of mutations at 54/57 (TACATTGG) resulted in a mutant Pm promoter (Pm 245) that had no activity at all (Table I).

At the 54/57 block some substitutions had little effect on XylS-dependent transcription from Pm (i.e. Pm 213, 11% reduction), whereas other substitutions at the same block had a clear effect (i.e. Pm 212, 78% reduction). In Pm 244, we combined the block of mutations in Pm 213 with a mutant block that had a moderate effect on the level of expression from Pm (i.e. 67/70 (TGCA  ACGT)). This combination had cumulative effects (Table I). These results confirmed the essential role of the nucleotides at these positions in XylS-dependent transcription activation from Pm.

The transcription initiation point of the mRNA generated from a number of the above Pm mutants promoters was the same that that determined for the wild-type promoter (not shown). This suggests that the mutations analyzed affected the strength of transcription from the mutant promoters.

Single Point Mutations within the Set of Blocks That Showed or Did Not Show Any Effect on Transcription Activation from Pm-- The above series of assays suggested critical, less critical, and irrelevant blocks for the XylS-dependent transcription activation from Pm. We expected that single mutations in irrelevant sets of sequences would have no effect at all on transcriptional activity from Pm, whereas single mutations in the critical and important blocks of sequences would have an effect. A number of bases at the noncritical region were selected to introduce single point mutations: A⁵¹  G, A⁶⁴  C, G⁶⁵  A, A⁷⁵  C, C⁷⁷  G, and T⁷⁸  G. These mutations, as expected, had little or no effect (<20%) on XylS-dependent transcription from Pm (not shown).

Of the less critical boxes (41/44, 54/57, 58/61, and 67/70), some of the substitutions (for example, A⁴²  T, A⁴⁴  G, A⁵⁴  G, A⁵⁴  T, G⁶⁹  C, and G⁶⁹  T) did not significantly affect activity from Pm (Table II); others had an intermediate effect, reducing the induced XylS-dependent activation of Pm by 20-50%. This effect was observed for the changes C⁵⁵  A, C⁵⁵  G, A⁶⁷  C, and A⁶⁷  T (Table II). Within these sequences, the change T⁷⁰  G resulted in loss of almost 90% of the activity (Table II).

In the 58/61 box, the G⁵⁹  C, G⁶⁰  T, and G⁶⁰  A changes had a significant effect on transcription, as shown by the finding that -galactosidase activity was less than 20% of that seen with the wild-type Pm promoter. A surprising finding was that the G⁶²  C change resulted in a mutant Pm promoter that lacked activity.

In the critical 46/49 set of bases, single bp substitutions had a significant effect on activity. The changes A⁴⁶  T, C⁴⁷  G or C⁴⁷  T, G⁴⁸  C, and T⁴⁹  G resulted in a 60-85% decrease in activity. However, the change G⁴⁸  A had little or no effect on transcriptional activity (Table II).

Role of the Bases at 54/57-- When we compared the critical sequences proposed by Kessler et al. (6) and Gallegos et al. (5), we found that they had the two TGCA submotifs at 46/49 and 67/70 in common (Fig. 1). The hypothesis of Gallegos et al. (5) suggested that the TACA submotif between 54/57 was critical for transcription activation from Pm, whereas Kessler et al. (6) suggested that only the 57/56 nucleotides were of importance (Fig. 1). Our results with single point mutations in the 54 to 57 region showed some effect in some positions, but in no case was the effect large enough to fully impede transcription (Table II). To elucidate the possible role of these four nucleotides, we generated the set of changes involving the 54/55, 55/56, 56/57, and 54/57 positions. We found that the 54/55 changes (CA  AC or CA  GT) did not influenced the level of -galactosidase activity from the mutant Pm promoters. In contrast, the 57/56 TA  AT change almost completely prevented activity (99% decrease). The combination of mutations at 55/56 and 54/57 had an intermediate effect, with activities in the range of 15-38% when the mutation involved the 57 position, and in the range of 40-80% when the changed involved the 56 position (not shown). From these results, we deduced that the 57/56 bases are critical for the activity of Pm.

	DISCUSSION

The XylS protein (12-14) belongs to the AraC/XylS family of regulators that comprises more than 100 different proteins involved in transcription stimulation of several cell processes, such as carbon metabolism, pathogenesis, and response to alkylating agents in bacteria (11). Members of the family for which in vitro or in vivo footprinting assays are available (AraC, RhaR, RhaS, MelR, SoxS, and Ada) have at least two features in common: they function in vivo as a dimer, and the stretch of nucleotides covered by a monomer of the regulatory protein at the regulated promoter is between 15 and 20 bp long. However, within this set of bases, short motifs seem to confer critical base recognition for DNA-protein interactions (15-21). This seems also to be the case for Pm/XylS interactions. Our in vivo footprinting analysis and the analysis of transcriptional activity from wild-type and mutant Pm promoters showed that critical nucleotides extended from the 70 position in the 5'-end to at least the 41 position in the 3' end. Our mutational analysis revealed that within this stretch the XylS recognition sequence seems to be TGCAN₆GGNTA, which appears twice in the Pm promoter, between 70 and 56 and between 49 and 35 (Fig. 1). In favor of this proposal is the observation that a tagged XylS-protein immunoadsorbed onto glass beads produced footprints in vitro, which showed protection of the Gs within the above direct repeat at 48, 59, 60, and 69 (22). This is in agreement with our in vivo results.

The AraC proteinthe best characterized regulator of the familystimulates transcription as a dimer (23-25). The consensus sequence for AraC-activable promoters is a direct repetition of the TAGCN₇TCCATA motif: each AraC monomer recognizes one of the direct repeats (26-29).

At the C-terminal end, the regulators of the AraC/XylS family show a highly conserved stretch of about 100 amino acids that seems to be involved in DNA binding and probably in interactions with RNA polymerase (11). One characteristic of members of this family is that they exhibit two possible HTH DNA binding motifs (located at 228-251 and 281-305 in XylS and at 198-217 and 246-264 in AraC). Brunelle and Schleif (15) analyzed these possible HTH motifs with substitutions of several amino acids that should contact DNA in an HTH structure and found evidence for the first one. Niland et al. (30), using synthetic oligonucleotides, systematically substituted the bases in the AraC recognition sequence and did gel retardation assays with mutant AraC in each of the possible HTH elements. They showed that the mutant AraC in each of the HTH motifs exhibited altered DNA binding properties. On the basis of their results, these authors proposed that each AraC monomer binds the 5'-TAGC submotif with one of the HTH motifs, and the 3'-TCCATA submotif with the second HTH.

If XylS contacts DNA via the two possible HTH elements, all mutant Pm promoters generated in different laboratories can be explained by the following model (Fig. 3): the XylS protein recognizes two submotifs in Pm, TGCA and GGNTA, which are separated from each other by six nucleotides. Each submotif is recognized by the recognition helix of one of the HTH elements of XylS. For the wild-type protein, recognition of direct sequences leads to the formation of a dimer. One monomer recognizes the upstream motif (from 70 to 56) with the two HTH DNA binding elements; the second monomer recognizes mainly the TGCA submotif and interacts with the downstream sequence, where it may compete for binding with the RNA-polymerase (Fig. 3). Mutations at the 46/49 TGCA submotif result in mutants in which the capacity to activate transcription is impaired because they cannot be contacted properly by XylS. Failure of one of the XylS monomers to interact with this submotif prevents dimer formation and leads to a nonactivable mutant Pm promoter. Pm mutants at the distal TGCA (67/70) submotif can be activated weakly by the wild-type XylS protein as a result of the formation of an unstable dimer; however, they can still be induced to a high level of activation by mutant XylS proteins with higher affinity for target sequences than the wild-type regulator (5). This is because one of the XylS mutant monomers binds to the downstream motif (49/35); and because of the mutation in the XylS, the second monomer is still able to interact well with the GGNTA submotif, and this suffices for dimer formation. The transcriptional activity of mutant Pm promoters with altered GGNTA at 60/56 sequences is seriously impaired, because failure of one of the monomers to bind correctly prevents dimer stabilization at Pm.

View larger version (9K): [in this window] [in a new window]   Fig. 3.   Diagramatic representation of XylS-Pm interactions.

	FOOTNOTES

^* This study was supported by European Commission Grant BIO-CT97-2313 and Comisión Interministerial de Ciencia y Tecnología of Spain Grant BIO97-0641.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed. Tel.: 34-958-121011; Fax:+34-958-129600; E-mail: jlramos{at}eez.csic.es.

The abbreviations used are: bp, base pair(s); HTH, helix-turn-helix; 3MB, 3-methylbenzoate; Pm, promoter for the TOL meta-pathway.

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				B. Carl and S. Fetzner Transcriptional Activation of Quinoline Degradation Operons of Pseudomonas putida 86 by the AraC/XylS-Type Regulator OxoS and Cross-Regulation of the PqorM Promoter by XylS Appl. Envir. Microbiol., December 1, 2005; 71(12): 8618 - 8626. [Abstract] [Full Text] [PDF]

				D. Tropel and J. R. van der Meer Bacterial Transcriptional Regulators for Degradation Pathways of Aromatic Compounds Microbiol. Mol. Biol. Rev., September 1, 2004; 68(3): 474 - 500. [Abstract] [Full Text] [PDF]

				M.-E. Guazzaroni, W. Teran, X. Zhang, M.-T. Gallegos, and J. L. Ramos TtgV Bound to a Complex Operator Site Represses Transcription of the Promoter for the Multidrug and Solvent Extrusion TtgGHI Pump J. Bacteriol., May 15, 2004; 186(10): 2921 - 2927. [Abstract] [Full Text] [PDF]

				S. Cunnac, C. Boucher, and S. Genin Characterization of the cis-Acting Regulatory Element Controlling HrpB-Mediated Activation of the Type III Secretion System and Effector Genes in Ralstonia solanacearum J. Bacteriol., April 15, 2004; 186(8): 2309 - 2318. [Abstract] [Full Text] [PDF]

				R. Ruiz, S. Marques, and J. L. Ramos Leucines 193 and 194 at the N-Terminal Domain of the XylS Protein, the Positive Transcriptional Regulator of the TOL meta-Cleavage Pathway, Are Involved in Dimerization J. Bacteriol., May 15, 2003; 185(10): 3036 - 3041. [Abstract] [Full Text] [PDF]

				J. A. Ibarra, M. I. Villalba, and J. L. Puente Identification of the DNA Binding Sites of PerA, the Transcriptional Activator of the bfp and per Operons in Enteropathogenic Escherichia coli J. Bacteriol., May 1, 2003; 185(9): 2835 - 2847. [Abstract] [Full Text] [PDF]

				S. M. Egan Growing Repertoire of AraC/XylS Activators J. Bacteriol., October 15, 2002; 184(20): 5529 - 5532. [Full Text] [PDF]

				R. Ruiz and J. L. Ramos Residues 137 and 153 at the N Terminus of the XylS Protein Influence the Effector Profile of This Transcriptional Regulator and the sigma Factor Used by RNA Polymerase to Stimulate Transcription from Its Cognate Promoter J. Biol. Chem., February 22, 2002; 277(9): 7282 - 7286. [Abstract] [Full Text] [PDF]

				E. Diaz, A. Ferrandez, M. A. Prieto, and J. L. Garcia Biodegradation of Aromatic Compounds by Escherichia coli Microbiol. Mol. Biol. Rev., December 1, 2001; 65(4): 523 - 569. [Abstract] [Full Text] [PDF]

				S. M. Egan, A. J. Pease, J. Lang, X. Li, V. Rao, W. K. Gillette, R. Ruiz, J. L. Ramos, and R. E. Wolf Jr. Transcription Activation by a Variety of AraC/XylS Family Activators Does Not Depend on the Class II-Specific Activation Determinant in the N-Terminal Domain of the RNA Polymerase Alpha Subunit J. Bacteriol., December 15, 2000; 182(24): 7075 - 7077. [Abstract] [Full Text]

				C. E. Cowles, N. N. Nichols, and C. S. Harwood BenR, a XylS Homologue, Regulates Three Different Pathways of Aromatic Acid Degradation in Pseudomonas putida J. Bacteriol., November 15, 2000; 182(22): 6339 - 6346. [Abstract] [Full Text]

				N. Kaldalu, U. Toots, V. de Lorenzo, and M. Ustav Functional Domains of the TOL Plasmid Transcription Factor XylS J. Bacteriol., February 15, 2000; 182(4): 1118 - 1126. [Abstract] [Full Text]

				P. M. Bhende and S. M. Egan Amino Acid-DNA Contacts by RhaS: an AraC Family Transcription Activator J. Bacteriol., September 1, 1999; 181(17): 5185 - 5192. [Abstract] [Full Text]

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