Selective stimulation of translation of leaderless mRNA by initiation factor 2: evolutionary implications for translation

Translation initiation in bacteria involves a stochastic binding mechanism in which the 30S ribosomal subunit first binds either to mRNA or to initiator tRNA, fMet-tRNA(f)(Met). Leaderless lambda cI mRNA did not form a binary complex with 30S ribosomes, which argues against the view that ribosomal recruitment signals other than a 5'-terminal start codon are essential for translation initiation of these mRNAs. We show that, in Escherichia coli, translation initiation factor 2 (IF2) selectively stimulates translation of lambda cI mRNA in vivo and in vitro. These experiments suggest that the start codon of leaderless mRNAs is recognized by a 30S-fMet-tRNA(f)(Met)-IF2 complex, an intermediate equivalent to that obligatorily formed during translation initiation in eukaryotes. We further show that leaderless lambda cI mRNA is faithfully translated in vitro in both archaebacterial and eukaryotic translation systems. This suggests that translation of leaderless mRNAs reflects a fundamental capability of the translational apparatus of all three domains of life and lends support to the hypothesis that the translation initiation pathway is universally conserved.     

Modified nucleotides in tRNA(Lys) and tRNA(Val) are important for translocation

Ribosomes translate genetic information encoded by messenger RNAs (mRNAs) into proteins. Accurate decoding by the ribosome depends on the proper interaction between the mRNA codon and the anticodon of transfer RNA (tRNA). tRNAs from all kingdoms of life are enzymatically modified at distinct sites, particularly in and near the anticodon. Yet, the role of these naturally occurring tRNA modifications in translation is not fully understood. Here we show that modified nucleosides at the first, or wobble, position of the anticodon and 3'-adjacent to the anticodon are important for translocation of tRNA from the ribosome's aminoacyl site (A site) to the peptidyl site (P site). Thus, naturally occurring modifications in tRNA contribute functional groups and conformational dynamics that are critical for accurate decoding of mRNA and for translocation to the P site during protein synthesis.     

Ribosomes bind leaderless mRNA in Escherichia coli through recognition of their 5'-terminal AUG

Leaderless mRNAs are translated in the absence of upstream signals that normally contribute to ribosome binding and translation efficiency. In order to identify ribosomal components that interact with leaderless mRNA, a fragment of leaderless cI mRNA from bacteriophage λ, with a 4-thiouridine (4^S-U) substituted at the +2 position of the AUG start codon, was used to form cross-links to Escherichia coli ribosomes during binary (mRNA+ribosome) and ternary (mRNA+ribosome+initiator tRNA) complex formation. Ribosome binding assays (i.e., toeprints) demonstrated tRNA-dependent binding of leaderless mRNA to ribosomes; however, cross-links between the start codon and 30S subunit rRNA and r-proteins formed independent of initiator tRNA. Toeprints revealed that a leaderless mRNA's 5′-AUG is required for stable binding. Furthermore, the addition of a 5′-terminal AUG triplet to a random RNA fragment can make it both competent and competitive for ribosome binding, suggesting that a leaderless mRNA's start codon is a major feature for ribosome interaction. Cross-linking assays indicate that a subset of 30S subunit r-proteins, located at either end of the mRNA tunnel, contribute to tRNA-independent contacts and/or interactions with a leaderless mRNA's start codon. The interaction of leaderless mRNA with ribosomes may reveal features of mRNA binding and AUG recognition that are distinct from known signals but are important for translation initiation of all mRNAs.     

GTP-independent tRNA Delivery to the Ribosomal P-site by a Novel Eukaryotic Translation Factor

During translation, aminoacyl-tRNAs are delivered to the ribosome by specialized GTPases called translation factors. Here, we report the tRNA binding to the P-site of 40 S ribosomes by a novel GTP-independent factor eIF2D isolated from mammalian cells. The binding of tRNA_i^Met occurs after the AUG codon finds its position in the P-site of 40 S ribosomes, the situation that takes place during initiation complex formation on the hepatitis C virus internal ribosome entry site or on some other specific RNAs (leaderless mRNA and A-rich mRNAs with relaxed scanning dependence). Its activity in tRNA binding with 40 S subunits does not require the presence of the aminoacyl moiety. Moreover, the factor possesses the unique ability to deliver non-Met (elongator) tRNAs into the P-site of the 40 S subunit. The corresponding gene is found in all eukaryotes and includes an SUI1 domain present also in translation initiation factor eIF1. The versatility of translation initiation strategies in eukaryotes is discussed.     

Protein Synthesis in E. coli: Dependence of Codon-Specific Elongation on tRNA Concentration and Codon Usage

To synthesize a protein, a ribosome moves along a messenger RNA (mRNA), reads it codon by codon, and takes up the corresponding ternary complexes which consist of aminoacylated transfer RNAs (aa-tRNAs), elongation factor Tu (EF-Tu), and GTP. During this process of translation elongation, the ribosome proceeds with a codon-specific rate. Here, we present a general theoretical framework to calculate codon-specific elongation rates and error frequencies based on tRNA concentrations and codon usages. Our theory takes three important aspects of in-vivo translation elongation into account. First, non-cognate, near-cognate and cognate ternary complexes compete for the binding sites on the ribosomes. Second, the corresponding binding rates are determined by the concentrations of free ternary complexes, which must be distinguished from the total tRNA concentrations as measured in vivo. Third, for each tRNA species, the difference between total tRNA and ternary complex concentration depends on the codon usages of the corresponding cognate and near-cognate codons. Furthermore, we apply our theory to two alternative pathways for tRNA release from the ribosomal E site and show how the mechanism of tRNA release influences the concentrations of free ternary complexes and thus the codon-specific elongation rates. Using a recently introduced method to determine kinetic rates of in-vivo translation from in-vitro data, we compute elongation rates for all codons in Escherichia coli. We show that for some tRNA species only a few tRNA molecules are part of ternary complexes and, thus, available for the translating ribosomes. In addition, we find that codon-specific elongation rates strongly depend on the overall codon usage in the cell, which could be altered experimentally by overexpression of individual genes.     

Footprinting mRNA-ribosome complexes with chemical probes.

We footprinted the interaction of model mRNAs with 30S ribosomal subunits in the presence or absence of tRNA(fMet) or tRNA(Phe) using chemical probes directed at the sugar-phosphate backbone or bases of the mRNAs. When bound to the 30S subunits in the presence of tRNA(fMet), the sugar-phosphate backbones of gene 32 mRNA and 022 mRNA are protected from hydroxyl radical attack within a region of about 54 nucleotides bounded by positions -35 (+/- 2) and +19, extending to position +22 when tRNA(Phe) is used. In 70S ribosomes, protection is extended in the 5' direction to about position -39 (+/- 2). In the absence of tRNA, the 30S subunit protects only nucleotides -35 (+/- 2) to +5. Introduction of a stable tetraloop hairpin between positions +10 and +11 of gene 32 mRNA does not interfere with tRNA(fMet)-dependent binding of the mRNA to 30S subunits, but results in loss of protection of the sugar-phosphate backbone of the mRNA downstream of position +5. Using base-specific probes, we find that the Shine-Dalgarno sequence (A-12, A-11, G-10 and G-9) and the initiation codon (A+1, U+2 and G+3) of gene 32 mRNA are strongly protected by 30S subunits in the presence of initiator tRNA. In the presence of tRNA(Phe), the same Shine-Dalgarno bases are protected, as are U+4, U+5 and U+6 of the phenylalanine codon. Interestingly, A-1, immediately preceding the initiation codon, is protected in the complex with 30S subunits and initiator tRNA, while U+2 and G+3 are protected in the complex with tRNA(Phe) in the absence of initiator tRNA. Additionally, specific bases upstream from the Shine-Dalgarno region (U-33, G-32 and U-22) as well as 3' to the initiation codon (G+11) are protected by 30S subunits in the presence of either tRNA. These results imply that the mRNA binding site of the 30S subunit covers about 54-57 nucleotides and are consistent with the possibility that the ribosome interacts with mRNA along its sugar-phosphate backbone.     

Non-standard translational events in Candida albicans mediated by an unusual seryl-tRNA with a 5'-CAG-3' (leucine) anticodon.

From in vitro translation studies we have previously demonstrated the existence of an apparent efficient UAG (amber) suppressor tRNA in the dimorphic fungus Candida albicans (Santos et al., 1990). Using an in vitro assay for termination codon readthrough the tRNA responsible was purified to homogeneity from C.albicans cells. The determined sequence of the purified tRNA predicts a 5'-CAG-3' anticodon that should decode the leucine codon CUG and not the UAG termination codon as originally hypothesized. However, the tRNA(CAG) sequence shows greater nucleotide homology with seryl-tRNAs from the closely related yeast Saccharomyces cerevisiae than with leucyl-tRNAs from the same species. In vitro tRNA-charging studies demonstrated that the purified tRNA(CAG) is charged with Ser. The gene encoding the tRNA was cloned from C.albicans by a PCR-based strategy and DNA sequence analysis confirmed both the structure of the tRNA(CAG) and the absence of any introns in the tRNA gene. The copy number of the tRNA(CAG) gene (1-2 genes per haploid genome) is in agreement with the relatively low abundance (< 0.5% total tRNA) of this tRNA. In vitro translation studies revealed that the purified tRNA(CAG) could induce apparent translational bypass of all three termination codons. However, peptide mapping of in vitro translation products demonstrated that the tRNA(CAG) induces translational misreading in the amino-terminal region of two RNA templates employed, namely the rabbit alpha- and beta-globin mRNAs. These results suggest that the C.albicans tRNA(CAG) is not an 'omnipotent' suppressor tRNA but rather may mediate a novel non-standard translational event in vitro during the translation of the CUG codon. The possible nature of this non-standard translation event is discussed in the context of both the unusual structural features of the tRNA(CAG) and its in vitro behaviour.     

Temperature-dependent translation of leaderless and canonical mRNAs in Escherichia coli

Leaderless mRNAs beginning with a 5'-terminal start codon occur in all biological systems. In this work, we have studied the comparative translational efficiency of leaderless and leadered mRNAs as a function of temperature by in vitro translation competition assays with Escherichia coli extracts. At low temperature (25 degrees C) leaderless mRNAs were found to be translated comparatively better than mRNAs containing an internal canonical ribosome binding site, whereas at high temperature (42 degrees C) the translational efficiency of canonical mRNAs is by far superior to that of leaderless mRNA. The inverse correlation between temperature and translational efficiency characteristic for the two mRNA classes was attributed to structural features of the mRNA(s) and to the reduced stability of the translation initiation complex formed at a 5'-terminal start codon at elevated temperature.     

Functional importance of RNA interactions in selection of translation initiation codons

RNA base pairing between the initiation codon and anticodon loop of initiator tRNA is essential but not sufficient for the selection of the 'correct' mRNA translational start site by ribosomes. In prokaryotes, additional RNA interactions between small ribosomal subunit RNA and mRNA sequences just upstream of the start codon can efficiently direct the ribosome to the initiation site. Although there is presently no proof for a similar important ribosomal RNA interaction in eukaryotes, the 5' non-coding regions of their mRNAs and 'consensus sequences' surrounding initiation codons have been shown to be strong determinants for initiation-site selection, but the exact mechanisms are not yet understood. Intramolecular base pairing in mRNA and participation of translation initiation factors can strongly influence the formation of mRNA–small ribosomal subunit–initiator tRNA complexes and modulate translational activities in both prokaryotes and eukaryotes. Only recently has it been appreciated that alternative mechanisms may also contribute to the selection of initiation codons in all organisms. Although direct proof is currently lacking, there is accumulating evidence that additional cis -acting mRNA elements and trans -acting proteins may form specific 'bridging' interactions with ribosomes during translation initiation.     

The role of tRNA and ribosome competition in coupling the expression of different mRNAs in Saccharomyces cerevisiae

Protein synthesis translates information from messenger RNAs into functional proteomes. Because of the finite nature of the resources required by the translational machinery, both the overall protein synthesis activity of a cell and activity on individual mRNAs are controlled by the allocation of limiting resources. Upon introduction of heterologous sequences into an organism-for example for the purposes of bioprocessing or synthetic biology-limiting resources may also become overstretched, thus negatively affecting both endogenous and heterologous gene expression. In this study, we present a mean-field model of translation in Saccharomyces cerevisiae for the investigation of two particular translational resources, namely ribosomes and aminoacylated tRNAs. We firstly use comparisons of experiments with heterologous sequences and simulations of the same conditions to calibrate our model, and then analyse the behaviour of the translational system in yeast upon introduction of different types of heterologous sequences. Our main findings are that: competition for ribosomes, rather than tRNAs, limits global translation in this organism; that tRNA aminoacylation levels exert, at most, weak control over translational activity; and that decoding speeds and codon adaptation exert strong control over local (mRNA specific) translation rates.     

Non-AUG initiation of AGAMOUS mRNA translation in Arabidopsis thaliana

The MADS box organ identity gene AGAMOUS (AG) controls several steps during Arabidopsis thaliana flower development. AG cDNA contains an open reading frame that lacks an ATG triplet to function as the translation initiation codon, and the actual amino terminus of the AG protein remains uncharacterized. We have considered the possibility that AG translation can be initiated at a non-AUG codon. Two possible non-AUG initiation codons, CUG and ACG, are present in the 5' region of AG mRNA preceding the highly conserved MADS box sequence. We prepared a series of AG genomic constructs in which these codons are mutated and assayed their activity in phenotypic rescue experiments by introducing them as transgenes into ag mutant plants. Alteration of the CTG codon to render it unsuitable for acting as a translation initiation site does not affect complementation of the ag-3 mutation in transgenic plants. However, a similar mutation of the downstream ACG codon prevents the rescue of the ag-3 mutant phenotype. Conversely, if an ATG is introduced immediately 5' to the disrupted ACG codon, the resulting construct fully complements the ag-3 mutation. The AG protein synthesized in vitro by initiating translation at the ACG position is active in DNA binding and is of the same size as the AG protein detected from floral tissues, whereas AG polypeptides with additional amino-terminal residues do not appear to bind DNA. These results indicate that translation of AG is initiated exclusively at an ACG codon and prove that non-AUG triplets may be efficiently used as the sole translation initiation site in some plant cellular mRNAs.     

tmRNA-induced release of messenger RNA from stalled ribosomes

A ribosome stalled on a truncated mRNA in the eubacterial cell can be rescued by tmRNA via a process called trans-translation. We demonstrate here that release of truncated mRNAs from stalled ribosomes accelerates significantly already after trans-peptidation following tmRNA binding to the ribosome. However, rapid release of truncated mRNA requires EF-G-dependent translocation of peptidyl-tmRNA from the A to the P site of the ribosome. We show also that the rate of mRNA release before and after peptidyl-tmRNA translocation correlates well with the rate of dissociation of deacylated tRNA, indicating that mRNA is retained on the ribosome mainly through codon:anticodon interaction with tRNA. The rate of mRNA release is reduced for mRNAs with strong Shine-Dalgarno (SD)-like sequences in the vicinity of the truncation site as well as for mRNAs with long 3' extensions downstream from the P-site codon. The reduced rate of release in the former case was due to a persisting SD-anti SD interaction between mRNA and the ribosome.     

Structural basis of translational control by Escherichia coli threonyl tRNA synthetase

Escherichia coli threonyl-tRNA synthetase (ThrRS) represses the translation of its own messenger RNA by binding to an operator located upstream of the initiation codon. The crystal structure of the complex between the core of ThrRS and the essential domain of the operator shows that the mRNA uses the recognition mode of the tRNA anticodon loop to initiate binding. The final positioning of the operator, upon which the control mechanism is based, relies on a characteristic RNA motif adapted to the enzyme surface. The finding of other thrS operators that have this conserved motif leads to a generalization of this regulatory mechanism to a subset of Gram-negative bacteria.     

Maintenance of the correct open reading frame by the ribosome

During translation, a string of non-overlapping triplet codons in messenger RNA is decoded into protein. The ability of a ribosome to decode mRNA without shifting between reading frames is a strict requirement for accurate protein biosynthesis. Despite enormous progress in understanding the mechanism of transfer RNA selection, the mechanism by which the correct reading frame is maintained remains unclear. In this report, evidence is presented that supports the idea that the translational frame is controlled mainly by the stability of codon–anticodon interactions at the P site. The relative instability of such interactions may lead to dissociation of the P-site tRNA from its codon, and formation of a complex with an overlapping codon, the process known as P-site tRNA slippage. We propose that this process is central to all known cases of +1 ribosomal frameshifting, including that required for the decoding of the yeast transposable element Ty3. An earlier model for the decoding of this element proposed 'out-of-frame' binding of A-site tRNA without preceding P-site tRNA slippage.     

Unfolding of mRNA secondary structure by the bacterial translation initiation complex

Translation initiation is a key step for regulating the level of numerous proteins within the cell. In bacteria, the 30S initiation complex directly binds to the translation initiation region (TIR) of the mRNA. How the ribosomal 30S subunit assembles on highly structured TIR is not known. Using fluorescence-based experiments, we assayed 12 different mRNAs that form secondary structures with various stabilities and contain Shine-Dalgarno (SD) sequences of different strengths. A strong correlation was observed between the stability of the mRNA structure and the association and dissociation rate constants. Interestingly, in the presence of initiation factors and initiator tRNA, the association kinetics of structured mRNAs showed two distinct phases. The second phase was found to be important for unfolding structured mRNAs to form a stable 30S initiation complex. We show that unfolding of structured mRNAs requires a SD sequence, the start codon, fMet-tRNA(fMet), and the GTP bound form of initiation factor 2 bound to the 30S subunit.     

Translation termination is involved in histone mRNA degradation when DNA replication is inhibited

The levels of replication-dependent histone mRNAs are coordinately regulated with DNA synthesis. A major regulatory step in histone mRNA metabolism is regulation of the half-life of histone mRNAs. Replication-dependent histone mRNAs are the only metazoan mRNAs that are not polyadenylated. Instead, they end with a conserved stem-loop structure, which is recognized by the stem-loop binding protein (SLBP). SLBP is required for histone mRNA processing, as well as translation. We show here, using histone mRNAs whose translation can be regulated by the iron response element, that histone mRNAs need to be actively translated for their rapid degradation following the inhibition of DNA synthesis. We also demonstrate the requirement for translation using a mutant SLBP which is inactive in translation. Histone mRNAs are not rapidly degraded when DNA synthesis is inhibited or at the end of S phase in cells expressing this mutant SLBP. Replication-dependent histone mRNAs have very short 3' untranslated regions, with the stem-loop located 30 to 70 nucleotides downstream of the translation termination codon. We show here that the stability of histone mRNAs can be modified by altering the position of the stem-loop, thereby changing the distance from the translation termination codon.     

Yeast initiator tRNA identity elements cooperate to influence multiple steps of translation initiation

All three kingdoms of life employ two methionine tRNAs, one for translation initiation and the other for insertion of methionines at internal positions within growing polypeptide chains. We have used a reconstituted yeast translation initiation system to explore the interactions of the initiator tRNA with the translation initiation machinery. Our data indicate that in addition to its previously characterized role in binding of the initiator tRNA to eukaryotic initiation factor 2 (eIF2), the initiator-specific A1:U72 base pair at the top of the acceptor stem is important for the binding of the eIF2.GTP.Met-tRNA(i) ternary complex to the 40S ribosomal subunit. We have also shown that the initiator-specific G:C base pairs in the anticodon stem of the initiator tRNA are required for the strong thermodynamic coupling between binding of the ternary complex and mRNA to the ribosome. This coupling reflects interactions that occur within the complex upon recognition of the start codon, suggesting that these initiator-specific G:C pairs influence this step. The effect of these anticodon stem identity elements is influenced by bases in the T loop of the tRNA, suggesting that conformational coupling between the D-loop-T-loop substructure and the anticodon stem of the initiator tRNA may occur during AUG codon selection in the ribosomal P-site, similar to the conformational coupling that occurs in A-site tRNAs engaged in mRNA decoding during the elongation phase of protein synthesis.