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J. Biol. Chem., Vol. 279, Issue 12, 11081-11087, March 19, 2004
Factors That Influence Selection of Coding Resumption Sites in Translational Bypassing
MINIMAL CONVENTIONAL PEPTIDYL-tRNA:mRNA PAIRING CAN SUFFICE*
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| ABSTRACT |
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| INTRODUCTION |
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Long range translational bypassing in response to A-site codons whose aminoacyl-tRNAs are limiting has been demonstrated (24). In these cases, the peptidyl-tRNA dissociates from the ribosomal P-site codon, scans the coding gap, and re-pairs to mRNA at complementary triplets. There are also examples of short distance hopping over stop codons (5), including
-globin in rabbits, where hopping produces some longer forms of the protein (6). In one of these studies, landing at sites with two out of three Watson-Crick or wobble pairs was reduced by at least two-thirds compared with a matched codon (7).
Long distance bypassing is required for expression of phage T4 gene 60 (8, 9) where 50 nucleotides are bypassed by 50% of the ribosomes that initiate translation (10). Several different signals in the mRNA are required. Most important for the current work is a nascent peptide sequence encoded upstream of the 50-nt1 coding gap that, while still within the ribosome, strongly stimulates take-off (9, 11, 12). Because the nascent peptide moves with the bypassing ribosome, the potential exists for this signal to also influence landing site selection. Forward ribosome slippage is normally constrained by ribosomal protein L9 (7, 13). In gene 60 bypassing the role of L9 is overridden through effects of a stem-loop structure at the 5' end of the coding gap (14). Deletion of the L9 gene reduces the constraints on forward mRNA slippage and increases the proportion of bypass products relative to termination and stop codon readthrough or frameshift products (7, 13). How L9 exerts its effect on restraining mRNA slippage is unclear. Its N-terminal domain binds within domain V of 23 S rRNA close to the base of the L1 stalk (15, 16). The L1 stalk is actively involved in the translocational movement of tRNA from the P-site to the E-site (17). L9 has a centrally located long
-helix (18) and a C-terminal region that undergoes substantial movement between two states in the ribosome cycle (19, 20). Strains with either WT L9 or lacking L9 were used in the current study, which explores factors that influence bypassing ribosomes to resume coding.
| EXPERIMENTAL PROCEDURES |
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-expressing strain {CSH142(sufSU34C)} has been described (22) as well as another isogenic strain containing a precise deletion of the gene encoding L9 {AH156} (13). The parental expression vectors for the GST-MBP-6XHis constructs (GTM10) and TrxA-6XHis-MBP construct (TSM1) have been described (7). The insert for the nascent peptide fusion construct was constructed using standard PCR techniques. For all other constructs, complementary oligonucleotides were designed to generate compatible ends for the appropriate restriction sites.
Protein AnalysisOvernight cultures of strains expressing the GST-MBP-6XHis plasmids were diluted 1:100 in Terrific Broth containing 100 µg/ml ampicillin, grown for 2 h at 37 °C, and then induced with 1 mM isopropyl-
-D-thiogalactoside for an additional 4 h at 37 °C. Harvested cells were lysed using Novagen BugBuster reagent. Full-length GST-MBP-6XHis fusion protein was purified by sequential passages over glutathione-Sepharose (Amersham Biosciences) and nickel-nitrilotriacetic acid-agarose (Qiagen). Cultures containing the TrxA-6XHis-gene 60-MBP plasmids were grown as described above except the inductions were carried out at 20 °C to maximize solubility of the gene 60 nascent peptide-containing fusion proteins. Full-length TrxA-6XHis-gene 60-MBP fusion protein was purified by sequential passages over nickel-nitrilotriacetic acid-agarose (Qiagen) and amylose-agarose (New England Biolabs). Eluted proteins were concentrated and washed extensively with Nanopure H2O using a Centricon 30 (Millipore) filtration unit. Protein was digested with PreScission Protease according to the protocol provided by the supplier (Amersham Biosciences). The digestion buffer was 50 mM Tris-HCl, pH 7.0, 150 mM NaCl, 1 mM EDTA, and 1 mM dithiothreitol. The samples contained the proteins of interest, as well as the GST- or TrxA-containing N-terminal portions of the fusion proteins, PreScission protease, and nonspecific contaminants that copurified on the affinity columns. For Fig. 1, protein concentrations were determined by measuring absorbance at 280 nm.
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Pulse-Chase AnalysisThe overall efficiency of bypassing, read-through, and/or frameshifting in a particular construct was determined essentially as described (22). Cells containing the GST-MBP-6His constructs were grown to mid-log phase in MOPS/glucose (23) including all amino acids (150 µg/ml) except methionine. Expression was induced by 2 mM isopropyl-
-D-thiogalactoside for 10 min. [35S]Methionine was added and the incubation continued for 2 min. Radioactivity was chased by the addition of excess cold methionine for 2 min. Total protein was separated on a 15% Tris-glycine SDS-polyacrylamide gel and visualized with a PhosphorImager from Amersham Biosciences. Efficiency estimates represent the amount of recoding product divided by the total protein synthesized from the fusion construct (recoding plus termination products) taking into account the differences in methionyl content between the products.
-Galactosidase AssaysThe bypass sequence for the construct shown in Fig. 4 was cloned in a vector between GST and lacZ such that expression of lacZ required frameshifting or bypassing into the +1 frame. Assays were performed as described (21) except that cell were grown in MOPS medium (23) supplemented with 20 amino acids.
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| RESULTS |
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Bypassing Can Lead to Coding Resumption at Multiple SitesPrevious work has shown that in-frame stop codons can promote peptidyl-tRNA bypassing. To explore the latitude in pairing requirements for coding resumption, a sequence containing few codons with potential for strong Watson-Crick interactions with peptidyl-tRNA was placed 3' of GGA UAA (Fig. 2A). GGA is the take-off codon for bacteriophage T4 gene 60 bypassing and is decoded by
(anticodon = 3'CCU*5' where the * is an unknown modification) (9, 22). None of the other stimulatory elements for gene 60 bypassing (the nascent peptide, the stem-loop in the coding gap, or matched landing codons) was included in this construct. Fusion protein resulting from readthrough and/or bypassing was purified, digested with PreScission protease, and then analyzed by electrospray mass spectrometry to characterize the translational event(s).
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Effect of Gene 60 Nascent PeptideAn effect of the gene 60 nascent peptide on dissociation of peptidyl-tRNA:mRNA pairing to initiate bypassing is known. Whether it also plays a role in landing site selection merited investigation. In the constructs designed, competition was set up between +1 frameshifting and bypassing, both of which require forward mRNA movement (Fig. 3A). These constructs contained thioredoxin A sequences in place of GST in order to increase the solubility of the highly insoluble gene 60 nascent peptide-containing products. In the control lacking the nascent peptide signal in a wild type L9 strain, +1 frameshifting from GGA to GAU, product 12, predominates, although there is bypassing to +22 CGU, product 14, +31 GAA, product 15, and +37 GGU, product 17 (Fig. 3, B and C). When the nascent peptide is present in a wild type L9 strain, +1 frame-shifting is eliminated, whereas bypassing to +49 GGU, product 19, is the most frequent event. These results could be because of the increased stringency requirement for re-pairing in the P-site of the nascent peptide; bypassing to +49 GGU allows for two Watson-Crick pairs, whereas +1 frameshifting to GAU allows for only a single, first position pair. Alternatively, these results could implicate the nascent peptide signal in promoting long distance mRNA slippage or an effect of landing site context. In the strain lacking L9 and the nascent peptide, where overall net shifting to the +1 frame is increased, bypassing is favored over +1 frame-shifting, as expected. The diversity of landing sites was also increased as revealed by landing at +13 AAU, product 13, +34 GAA, product 16, and +49 GGU, product 19 (Fig. 3, B and C). However, in the strain lacking L9 but containing the nascent peptide, the predominant landing site, +49 GGU, did not change, although the distribution of the lesser products was altered.
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Nascent Peptide Effect on Forward mRNA MovementTo test directly for the ability of the peptide signal to affect distant forward message movement, two constructs were designed that contained similar downstream landing sites. The take-off site was changed to GGG UAG to allow the +1 frameshifting site (underlined) to duplicate the downstream landing sites, GGU. Many of the suboptimal landing sites observed in Fig. 3 were changed to less favorable sites to more easily monitor landing at GGU. In this experiment, the immediate context of the downstream GGU landing sites was the same, CAA-5' and AAA-3', to preclude adjacent sequence effects on landing. A strain containing
with a mutation of its anticodon base 34, U*, to C (22, 24, 25) was utilized because its anticodon is complementary to the modified take-off site, GGG. The strain was also WT for L9. In the construct lacking the nascent peptide signal, there is +1 frameshifting, product 20, as well as bypassing to the closer +22 GGU, product 21, and a trace amount to the more distant +49 GGU, product 22 (Fig. 4, B and C). When the nascent peptide signal is present, only the +1 frameshifting product, 20, is detected. The presence of the nascent peptide increased transition to the +1 frame by 30-fold when assayed in a lacZ reporter system (from 0.3 to 9
-galactosidase units). It follows that the nascent peptide does not promote distant forward mRNA movement, but rather imposes a greater stringency requirement for P-site codon:anticodon pairing.
Shine-Dalgarno Effect on Peptidyl-tRNA Landing Site SelectionIn Figs. 2 and 3, particular landing sites may be utilized because of an mRNA signal that could direct landing. One such element known to affect framing during translation is a Shine-Dalgarno-like sequence (reviewed in Ref. 26). In these constructs, the take-off site, GGA, itself a weak Shine-Dalgarno sequence, could be affecting landing, in particular, at AAU with no potential for Watson-Crick or wobble pairing. To test directly the effect of a Shine-Dalgarno sequence on landing, a take-off site lacking As or Gs was utilized, and a Shine-Dalgarno sequence was introduced downstream. The take-off site, chosen from a set of all possible matched codons for the gene 60 take-off and landing sites,2 was UCC. The constructs shown in Fig. 5 contain a suboptimal UAC landing site 17 nt 3' of the take-off site and an optimal UCC landing site 29 nt 3' of the take-off site (Fig. 5A). In one construct, a Shine-Dalgarno-like sequence (GGAGG) is positioned 6 nt 5' of the suboptimal UAC. In the other construct, this Shine-Dalgarno-like sequence is mutated to CUACU. Because take-off is less efficient with UCC, detailed product analysis was undertaken only in an L9-deficient strain that favors forward mRNA slippage. In the presence of the Shine-Dalgarno-like sequence, landing occurred at UAC by two out of three pairings (Fig. 5, product 23), at AAC by one out of three pairings (product 24), and at the matched UCC (product 25). In the absence of the Shine-Dalgarno-like sequence, landing was detected only at the matched UCC codon (product 25). Based on previous proof of the effect of Shine-Dalgarno-like mRNA sequences and 16 S rRNA interactions on frameshifting (27, 28) and the present results, it was concluded that Shine-Dalgarno:16 S rRNA interactions can exert a significant influence on landing site selection.
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| DISCUSSION |
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The potential of the rRNA in ribosomes to pair with Shine-Dalgarno sequences in mRNAs as they are translating was discovered through its consequences for both +1 and 1 frameshifting (27, 28). Knowing this potential, it is perhaps not surprising that Shine-Dalgarno sequences can also influence landing site selection by bypassing ribosomes because they linearly scan mRNA in the absence of special mRNA signals (4). Nevertheless, it is new, and its implications merit consideration. When a Shine-Dalgarno sequence is located close to a gene terminator on its 3' side, it will influence landing site selection by ribosomes that bypass that terminator. The equivalent effect is expected with "hungry" codons where aminoacylated tRNA is limited, because they also promote bypassing (2). Shine-Dalgarno sequences also occur 5' of gene terminators when decoding of a flanking gene is coupled with that of the upstream gene (29). The 30 S subunit, which contains the anti-Shine-Dalgarno sequence, is likely tethered to the mRNA via an interaction with the Shine-Dalgarno with facilitation of initiation (30). However, Shine-Dalgarno sequences thus positioned 5' of gene terminators will also be expected, as in the frameshifting required for release factor 2 synthesis (31), to favor forward mRNA movement and thereby destabilize P-site pairing. Bypassing could also be initiated with landing influenced by the Shine-Dalgarno as well. For bypassing to be stimulated by a Shine-Dalgarno sequence, the optimal spacing from the take-off site should be 3 nt 5' which could direct landing on the stop codon located 6 nt 3'. Whether mRNA interactions with the rRNA of translating ribosomes will be used elsewhere for recoding remains to be seen, but 5' stimulators are also known for mammalian frameshifting (11, 32).
The present work shows that peptidyl transfer can proceed in vivo in the absence of Watson-Crick or wobble P-site anticodon:codon pairing as has been shown previously to occur in vitro at extremely low levels (33, 34). This is most likely not the general case for bypassing, however, because the zero out of three landing observed here was most likely stimulated by an upstream Shine-Dalgarno sequence, as discussed above. However, P-site pairing may not be necessary in other situations including some cases of programmed 1 frameshifting. In classical tandem 1 frameshifting, re-pairing occurs in both the A- and P-sites (35). However, studies of bacteriophage T7 gene 10 (36), derivatives of the coronavirus-like equine arteritis virus (37), and recently, hexanucleotide shift sites (38, 39) revealed cases where re-pairing by P-site tRNA was problematic. One model to resolve this difficulty is that mRNA repositioning occurs without the absolute need for peptidyl-tRNA pairing.
In gene 60 bypassing, the nascent peptide exerts its effect of destabilizing pairing in the P-site while still within the ribosome, presumably in the exit tunnel. Previous work has shown effects of nascent peptide sequences on inhibition of peptidyl-tRNA transfer or cleavage or translation elongation in decoding of various genes including tryptophanase, the upstream open reading frame preceding the coding region of mammalian S-adenosylmethionine decarboxylase, a human cytomegalovirus gene and a fungal arg gene (4042). The present finding of a nascent peptide effect on the fidelity of codon-anticodon interaction in the ribosomal P-site strengthens appreciation of the versatile effects that nascent peptides can have on decoding.
| FOOTNOTES |
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The on-line version of this article (available at http://www.jbc.org) contains Figs. 14. ![]()
Both authors contributed equally to this work and should be considered as joint first authors. ![]()
Supported by National Institutes of Health Genetics Training Grant 5T32GM07464-24. Present address: Sainsbury Laboratory, John Innes Centre, Norwich, NR4 7UH, United Kingdom. ![]()
¶ To whom correspondence should be addressed: Dept. of Human Genetics, University of Utah, 15 N. 2030 E., Salt Lake City, UT 84112-5330. Tel.: 801-585-3434; Fax: 801-585-3910; E-mail: john.atkins{at}genetics.utah.edu.
1 The abbreviations used are: nt, nucleotide; MBP, maltose-binding protein; GST, glutathione S-transferase; MOPS, 4-morpholinepropane-sulfonic acid; WT, wild type; TrxA, thioredoxin A. ![]()
2 D. Bucklin, N. Wills, R. Gesteland, and J. Atkins, manuscript in preparation. ![]()
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