Reading frame selection and transfer RNA anticodon loop stacking

Tuning tRNAs for improved translation

Transfer RNAs have been extensively explored as the molecules that translate the genetic code into proteins. At this interface of genetics and biochemistry, tRNAs direct the efficiency of every major step of translation by interacting with a multitude of binding partners. However, due to the variability of tRNA sequences and the abundance of diverse post-transcriptional modifications, a guidebook linking tRNA sequences to specific translational outcomes has yet to be elucidated. Here, we review substantial efforts that have collectively uncovered tRNA engineering principles that can be used as a guide for the tuning of translation fidelity. These principles have allowed for the development of basic research, expansion of the genetic code with non-canonical amino acids, and tRNA therapeutics.

Engineering tRNAs for the Ribosomal Translation of Non-proteinogenic Monomers

Ribosome-dependent protein biosynthesis is an essential cellular process mediated by transfer RNAs (tRNAs). Generally, ribosomally synthesized proteins are limited to the 22 proteinogenic amino acids (pAAs: 20 l-α-amino acids present in the standard genetic code, selenocysteine, and pyrrolysine). However, engineering tRNAs for the ribosomal incorporation of non-proteinogenic monomers (npMs) as building blocks has led to the creation of unique polypeptides with broad applications in cellular biology, material science, spectroscopy, and pharmaceuticals. Ribosomal polymerization of these engineered polypeptides presents a variety of challenges for biochemists, as translation efficiency and fidelity is often insufficient when employing npMs. In this Review, we will focus on the methodologies for engineering tRNAs to overcome these issues and explore recent advances both in vitro and in vivo. These efforts include increasing orthogonality, recruiting essential translation factors, and creation of expanded genetic codes. After our review on the biochemical optimizations of tRNAs, we provide examples of their use in genetic code manipulation, with a focus on the in vitro discovery of bioactive macrocyclic peptides containing npMs. Finally, an analysis of the current state of tRNA engineering is presented, along with existing challenges and future perspectives for the field.

Phylogenomics and plastomics offer new evolutionary perspectives on Kalanchoideae (Crassulaceae)

BACKGROUND AND AIMS

Kalanchoideae is one of three subfamilies within Crassulaceae and contains four genera. Despite previous efforts, the phylogeny of Kalanchoideae remains inadequately resolved with persistent issues including low support, unstructured topologies and polytomies. This study aimed to address two central objectives: (1) resolving the pending phylogenetic questions within Kalanchoideae by using organelle-scale 'barcodes' (plastomes) and nuclear data; and (2) investigating interspecific diversity patterns among Kalanchoideae plastomes.

METHODS

To explore the plastome evolution in Kalanchoideae, we newly sequenced 38 plastomes representing all four constituent genera (Adromischus, Cotyledon, Kalanchoe and Tylecodon). We performed comparative analyses of plastomic features, including GC and gene contents, gene distributions at the IR (inverted repeat) boundaries, nucleotide divergence, plastomic tRNA (pttRNA) structures and codon aversions. Additionally, phylogenetic inferences were inferred using both the plastomic dataset (79 genes) and nuclear dataset (1054 genes).

KEY RESULTS

Significant heterogeneities were observed in plastome lengths among Kalanchoideae, strongly correlated with LSC (large single copy) lengths. Informative diversities existed in the gene content at SSC/IRa (small single copy/inverted repeat a), with unique patterns individually identified in Adromischus leucophyllus and one major Kalanchoe clade. The ycf1 gene was assessed as a shared hypervariable region among all four genera, containing nine lineage-specific indels. Three pttRNAs exhibited unique structures specific to Kalanchoideae and the genera Adromischus and Kalanchoe. Moreover, 24 coding sequences revealed a total of 41 lineage-specific unused codons across all four constituent genera. The phyloplastomic inferences clearly depicted internal branching patterns in Kalanchoideae. Most notably, by both plastid- and nuclear-based phylogenies, our research offers the first evidence that Kalanchoe section Eukalanchoe is not monophyletic.

CONCLUSIONS

This study conducted comprehensive analyses on 38 newly reported Kalanchoideae plastomes. Importantly, our results not only reconstructed well-resolved phylogenies within Kalanchoideae, but also identified highly informative unique markers at the subfamily, genus and species levels. These findings significantly enhance our understanding of the evolutionary history of Kalanchoideae.

Structural Diversities and Phylogenetic Signals in Plastomes of the Early-Divergent Angiosperms: A Case Study in Saxifragales

As representative of the early-divergent groups of angiosperms, Saxifragales is extremely divergent in morphology, comprising 15 families. Within this order, our previous case studies observed significant structural diversities among the plastomes of several lineages, suggesting a possible role in elucidating their deep phylogenetic relationships. Here, we collected 208 available plastomes from 11 constituent families to explore the evolutionary patterns among Saxifragales. With thorough comparisons, the losses of two genes and three introns were found in several groups. Notably, 432 indel events have been observed from the introns of all 17 plastomic intron-containing genes, which could well play an important role in family barcoding. Moreover, numerous heterogeneities and strong intrafamilial phylogenetic implications were revealed in pttRNA (plastomic tRNA) structures, and the unique structural patterns were also determined for five families. Most importantly, based on the well-supported phylogenetic trees, evident phylogenetic signals were detected in combinations with the identified pttRNAs features and intron indels, demonstrating abundant lineage-specific characteristics for Saxifragales. Collectively, the results reported here could not only provide a deeper understanding into the evolutionary patterns of Saxifragales, but also provide a case study for exploring the plastome evolution at a high taxonomic level of angiosperms.

Genome Expansion by tRNA +1 Frameshifting at Quadruplet Codons

Inducing tRNA +1 frameshifting to read a quadruplet codon has the potential to incorporate a non-canonical amino acid (ncAA) into the polypeptide chain. While this strategy is attractive for genome expansion in biotechnology and bioengineering endeavors, improving the yield is hampered by a lack of understanding of where the shift can occur in an elongation cycle of protein synthesis. Lacking a clear answer to this question, current efforts have focused on designing +1-frameshifting tRNAs with an extra nucleotide inserted to the anticodon loop for pairing with a quadruplet codon in the aminoacyl-tRNA binding (A) site of the ribosome. However, the designed and evolved +1-frameshifting tRNAs vary broadly in achieving successful genome expansion. Here we summarize recent work on +1-frameshifting tRNAs. We suggest that, rather than engineering the quadruplet anticodon-codon pairing scheme at the ribosome A site, efforts should be made to engineer the pairing scheme at steps after the A site, including the step of the subsequent translocation and the step that stabilizes the pairing scheme in the +1-frame in the peptidyl-tRNA binding (P) site.

Genetic Code Expansion Through Quadruplet Codon Decoding

Noncanonical amino acid mutagenesis has emerged as a powerful tool for the study of protein structure and function. While triplet nonsense codons, especially the amber codon, have been widely employed, quadruplet codons have attracted attention for the potential of creating additional blank codons for noncanonical amino acids mutagenesis. In this review, we discuss methodologies and applications of quadruplet codon decoding in genetic code expansion both in vitro and in vivo.

Multiplex suppression of four quadruplet codons via tRNA directed evolution

Genetic code expansion technologies supplement the natural codon repertoire with assignable variants in vivo, but are often limited by heterologous translational components and low suppression efficiencies. Here, we explore engineered Escherichia coli tRNAs supporting quadruplet codon translation by first developing a library-cross-library selection to nominate quadruplet codon-anticodon pairs. We extend our findings using a phage-assisted continuous evolution strategy for quadruplet-decoding tRNA evolution (qtRNA-PACE) that improved quadruplet codon translation efficiencies up to 80-fold. Evolved qtRNAs appear to maintain codon-anticodon base pairing, are typically aminoacylated by their cognate tRNA synthetases, and enable processive translation of adjacent quadruplet codons. Using these components, we showcase the multiplexed decoding of up to four unique quadruplet codons by their corresponding qtRNAs in a single reporter. Cumulatively, our findings highlight how E. coli tRNAs can be engineered, evolved, and combined to decode quadruplet codons, portending future developments towards an exclusively quadruplet codon translation system.

Mechanism of tRNA-mediated +1 ribosomal frameshifting

Accurate translation of the genetic code is critical to ensure expression of proteins with correct amino acid sequences. Certain tRNAs can cause a shift out of frame (i.e., frameshifting) due to imbalances in tRNA concentrations, lack of tRNA modifications or insertions or deletions in tRNAs (called frameshift suppressors). Here, we determined the structural basis for how frameshift-suppressor tRNA^SufA6 (a derivative of tRNA^Pro) reprograms the mRNA frame to translate a 4-nt codon when bound to the bacterial ribosome. After decoding at the aminoacyl (A) site, the crystal structure of the anticodon stem-loop of tRNA^SufA6 bound in the peptidyl (P) site reveals ASL conformational changes that allow for recoding into the +1 mRNA frame. Furthermore, a crystal structure of full-length tRNA^SufA6 programmed in the P site shows extensive conformational rearrangements of the 30S head and body domains similar to what is observed in a translocation intermediate state containing elongation factor G (EF-G). The 30S movement positions tRNA^SufA6 toward the 30S exit (E) site disrupting key 16S rRNA-mRNA interactions that typically define the mRNA frame. In summary, this tRNA-induced 30S domain change in the absence of EF-G causes the ribosome to lose its grip on the mRNA and uncouples the canonical forward movement of the tRNAs during elongation.

Miscoding-induced stalling of substrate translocation on the bacterial ribosome

Directional transit of the ribosome along the messenger RNA (mRNA) template is a key determinant of the rate and processivity of protein synthesis. Imaging of the multistep translocation mechanism using single-molecule FRET has led to the hypothesis that substrate movements relative to the ribosome resolve through relatively long-lived late intermediates wherein peptidyl-tRNA enters the P site of the small ribosomal subunit via reversible, swivel-like motions of the small subunit head domain within the elongation factor G (GDP)-bound ribosome complex. Consistent with translocation being rate-limited by recognition and productive engagement of peptidyl-tRNA within the P site, we now show that base-pairing mismatches between the peptidyl-tRNA anticodon and the mRNA codon dramatically delay this rate-limiting, intramolecular process. This unexpected relationship between aminoacyl-tRNA decoding and translocation suggests that miscoding antibiotics may impact protein synthesis by impairing the recognition of peptidyl-tRNA in the small subunit P site during EF-G-catalyzed translocation. Strikingly, we show that elongation factor P (EF-P), traditionally known to alleviate ribosome stalling at polyproline motifs, can efficiently rescue translocation defects arising from miscoding. These findings help reveal the nature and origin of the rate-limiting steps in substrate translocation on the bacterial ribosome and indicate that EF-P can aid in resuming translation elongation stalled by miscoding errors.

Synthetic biology approaches to biological containment: pre-emptively tackling potential risks

Biocontainment comprises any strategy applied to ensure that harmful organisms are confined to controlled laboratory conditions and not allowed to escape into the environment. Genetically engineered microorganisms (GEMs), regardless of the nature of the modification and how it was established, have potential human or ecological impact if accidentally leaked or voluntarily released into a natural setting. Although all evidence to date is that GEMs are unable to compete in the environment, the power of synthetic biology to rewrite life requires a pre-emptive strategy to tackle possible unknown risks. Physical containment barriers have proven effective but a number of strategies have been developed to further strengthen biocontainment. Research on complex genetic circuits, lethal genes, alternative nucleic acids, genome recoding and synthetic auxotrophies aim to design more effective routes towards biocontainment. Here, we describe recent advances in synthetic biology that contribute to the ongoing efforts to develop new and improved genetic, semantic, metabolic and mechanistic plans for the containment of GEMs.

An integrated, structure- and energy-based view of the genetic code

The principles of mRNA decoding are conserved among all extant life forms. We present an integrative view of all the interaction networks between mRNA, tRNA and rRNA: the intrinsic stability of codon-anticodon duplex, the conformation of the anticodon hairpin, the presence of modified nucleotides, the occurrence of non-Watson-Crick pairs in the codon-anticodon helix and the interactions with bases of rRNA at the A-site decoding site. We derive a more information-rich, alternative representation of the genetic code, that is circular with an unsymmetrical distribution of codons leading to a clear segregation between GC-rich 4-codon boxes and AU-rich 2:2-codon and 3:1-codon boxes. All tRNA sequence variations can be visualized, within an internal structural and energy framework, for each organism, and each anticodon of the sense codons. The multiplicity and complexity of nucleotide modifications at positions 34 and 37 of the anticodon loop segregate meaningfully, and correlate well with the necessity to stabilize AU-rich codon-anticodon pairs and to avoid miscoding in split codon boxes. The evolution and expansion of the genetic code is viewed as being originally based on GC content with progressive introduction of A/U together with tRNA modifications. The representation we present should help the engineering of the genetic code to include non-natural amino acids.

Systematic Evolution and Study of UAGN Decoding tRNAs in a Genomically Recoded Bacteria

We report the first systematic evolution and study of tRNA variants that are able to read a set of UAGN (N = A, G, U, C) codons in a genomically recoded E. coli strain that lacks any endogenous in-frame UAGN sequences and release factor 1. Through randomizing bases in anticodon stem-loop followed by a functional selection, we identified tRNA mutants with significantly improved UAGN decoding efficiency, which will augment the current efforts on genetic code expansion through quadruplet decoding. We found that an extended anticodon loop with an extra nucleotide was required for a detectable efficiency in UAGN decoding. We also observed that this crucial extra nucleotide was converged to a U (position 33.5) in all of the top tRNA hits no matter which UAGN codon they suppress. The insertion of U33.5 in the anticodon loop likely causes tRNA distortion and affects anticodon-codon interaction, which induces +1 frameshift in the P site of ribosome. A new model was proposed to explain the observed features of UAGN decoding. Overall, our findings elevate our understanding of the +1 frameshift mechanism and provide a useful guidance for further efforts on the genetic code expansion using a non-canonical quadruplet reading frame.

Codon-Anticodon Recognition in the Bacillus subtilis glyQS T Box Riboswitch: RNA-DEPENDENT CODON SELECTION OUTSIDE THE RIBOSOME

Many amino acid-related genes in Gram-positive bacteria are regulated by the T box riboswitch. The leader RNA of genes in the T box family controls the expression of downstream genes by monitoring the aminoacylation status of the cognate tRNA. Previous studies identified a three-nucleotide codon, termed the "Specifier Sequence," in the riboswitch that corresponds to the amino acid identity of the downstream genes. Pairing of the Specifier Sequence with the anticodon of the cognate tRNA is the primary determinant of specific tRNA recognition. This interaction mimics codon-anticodon pairing in translation but occurs in the absence of the ribosome. The goal of the current study was to determine the effect of a full range of mismatches for comparison with codon recognition in translation. Mutations were individually introduced into the Specifier Sequence of the glyQS leader RNA and tRNA(Gly) anticodon to test the effect of all possible pairing combinations on tRNA binding affinity and antitermination efficiency. The functional role of the conserved purine 3' of the Specifier Sequence was also verifiedin this study. We found that substitutions at the Specifier Sequence resulted in reduced binding, the magnitude of which correlates well with the predicted stability of the RNA-RNA pairing. However, the tolerance for specific mismatches in antitermination was generally different from that during decoding, which reveals a unique tRNA recognition pattern in the T box antitermination system.

Structural insights into translational recoding by frameshift suppressor tRNASufJ

The three-nucleotide mRNA reading frame is tightly regulated during translation to ensure accurate protein expression. Translation errors that lead to aberrant protein production can result from the uncoupled movement of the tRNA in either the 5' or 3' direction on mRNA. Here, we report the biochemical and structural characterization of +1 frameshift suppressor tRNA(SufJ), a tRNA known to decode four, instead of three, nucleotides. Frameshift suppressor tRNA(SufJ) contains an insertion 5' to its anticodon, expanding the anticodon loop from seven to eight nucleotides. Our results indicate that the expansion of the anticodon loop of either ASL(SufJ) or tRNA(SufJ) does not affect its affinity for the A site of the ribosome. Structural analyses of both ASL(SufJ) and ASL(Thr) bound to the Thermus thermophilus 70S ribosome demonstrate both ASLs decode in the zero frame. Although the anticodon loop residues 34-37 are superimposable with canonical seven-nucleotide ASLs, the single C31.5 insertion between nucleotides 31 and 32 in ASL(SufJ) imposes a conformational change of the anticodon stem, that repositions and tilts the ASL toward the back of the A site. Further modeling analyses reveal that this tilting would cause a distortion in full-length A-site tRNA(SufJ) during tRNA selection and possibly impede gripping of the anticodon stem by 16S rRNA nucleotides in the P site. Together, these data implicate tRNA distortion as a major driver of noncanonical translation events such as frameshifting.

Structural insights into +1 frameshifting promoted by expanded or modification-deficient anticodon stem loops

Maintenance of the correct reading frame on the ribosome is essential for accurate protein synthesis. Here, we report structures of the 70S ribosome bound to frameshift suppressor tRNA(SufA6) and N1-methylguanosine at position 37 (m(1)G37) modification-deficient anticodon stem loop(Pro), both of which cause the ribosome to decode 4 rather than 3 nucleotides, resulting in a +1 reading frame. Our results reveal that decoding at +1 suppressible codons causes suppressor tRNA(SufA6) to undergo a rearrangement of its 5' stem that destabilizes U32, thereby disrupting the conserved U32-A38 base pair. Unexpectedly, the removal of the m(1)G37 modification of tRNA(Pro) also disrupts U32-A38 pairing in a structurally analogous manner. The lack of U32-A38 pairing provides a structural correlation between the transition from canonical translation and a +1 reading of the mRNA. Our structures clarify the molecular mechanism behind suppressor tRNA-induced +1 frameshifting and advance our understanding of the role played by the ribosome in maintaining the correct translational reading frame.

Eight new mtDNA sequences of glass sponges reveal an extensive usage of +1 frameshifting in mitochondrial translation

Three previously studied mitochondrial genomes of glass sponges (phylum Porifera, class Hexactinellida) contained single nucleotide insertions in protein coding genes inferred as sites of +1 translational frameshifting. To investigate the distribution and evolution of these sites and to help elucidate the mechanism of frameshifting, we determined eight new complete or nearly complete mtDNA sequences from glass sponges and examined individual mitochondrial genes from three others. We found nine new instances of single nucleotide insertions in these sequences and analyzed them both comparatively and phylogenetically. The base insertions appear to have been gained and lost repeatedly in hexactinellid mt protein genes, suggesting no functional significance for the frameshifting sites. A high degree of sequence conservation, the presence of unusual tRNAs, and a distinct pattern of codon usage suggest the "out-of-frame pairing" model of translational frameshifting. Additionally, we provide evidence that relaxed selection pressure on glass sponge mtDNA - possibly a result of their low growth rates and deep-water lifestyle - has allowed frameshift insertions to be tolerated for hundreds of millions of years. Our study provides the first example of a phylogenetically diverse and extensive usage of translational frameshifting in animal mitochondrial coding sequences.

Molecular reconstruction of a fungal genetic code alteration

Fungi of the CTG clade translate the Leu CUG codon as Ser. This genetic code alteration is the only eukaryotic sense-to-sense codon reassignment known to date, is mediated by an ambiguous serine tRNA (tRNACAG(Ser)), exposes unanticipated flexibility of the genetic code and raises major questions about its selection and fixation in this fungal lineage. In particular, the origin of the tRNACAG(Ser) and the evolutionary mechanism of CUG reassignment from Leu to Ser remain poorly understood. In this study, we have traced the origin of the tDNACAG(Ser) gene and studied critical mutations in the tRNACAG(Ser) anticodon-loop that modulated CUG reassignment. Our data show that the tRNACAG(Ser) emerged from insertion of an adenosine in the middle position of the 5'-CGA-3'anticodon of a tRNACGA(Ser) ancestor, producing the 5'-CAG-3' anticodon of the tRNACAG(Ser), without altering its aminoacylation properties. This mutation initiated CUG reassignment while two additional mutations in the anticodon-loop resolved a structural conflict produced by incorporation of the Leu 5'-CAG-3'anticodon in the anticodon-arm of a tRNA(Ser). Expression of the mutant tRNACAG(Ser) in yeast showed that it cannot be expressed at physiological levels and we postulate that such downregulation was essential to maintain Ser misincorporation at sub-lethal levels during the initial stages of CUG reassignment. We demonstrate here that such low level CUG ambiguity is advantageous in specific ecological niches and we propose that misreading tRNAs are targeted for degradation by an unidentified tRNA quality control pathway.

Reprogramming the genetic code: from triplet to quadruplet codes

The genetic code of cells is near-universally triplet, and since many ribosomal mutations are lethal, changing the cellular ribosome to read nontriplet codes is challenging. Herein we review work on the incorporation of unnatural amino acids into proteins in response to quadruplet codons, and the creation of an orthogonal translation system in the cell that uses an evolved orthogonal ribosome to efficiently direct the incorporation of unnatural amino acids in response to quadruplet codons. Using this system multiple distinct unnatural amino acids have been incorporated and used to genetically program emergent properties into recombinant proteins. Extension of approaches to incorporate multiple unnatural amino acids may allow the combinatorial biosynthesis of materials and therapeutics, and drive investigations into whether life with additional genetically encoded polymers can evolve to perform functions that natural biological systems cannot.

Anticodon loop mutations perturb reading frame maintenance by the E site tRNA

The ribosomal E site helps hold the reading frame. Certain tRNA mutations affect translation, and anticodon loop mutations can be especially detrimental. We studied the effects of mutations saturating the anticodon loop of the amber suppressor tRNA, Su7, on the ability to help hold the reading frame when in the E site. We also tested three mutations in the anticodon stem, as well as a mutation in the D stem (the "Hirsh" mutation). We used the Escherichia coli RF2 programmed frameshift site to monitor frame maintenance. Most anticodon loop mutations increase frameshifting, possibly by decreasing codon:anticodon stability. However, it is likely that the A site is more sensitive to anticodon loop structure than is the E site. Unexpectedly, the Hirsh mutation also increases frameshifting from the E site. Other work shows that mutation may increase the ability of tRNA to react in the A site, possibly by facilitating conformational changes required for aminoacyl-tRNA selection. We suggest that this property may decrease its ability to bind to the E site. Finally, the absence of the ms(2)io(6)A nucleoside modifications at A37 does not decrease the ability of tRNA to help hold the reading frame from the E site. This was also unexpected because the absence of these modifications affects translational properties of tRNA in A and P sites. The absence of a negative effect in the E site further highlights the differences among the substrate requirements of the ribosomal coding sites.

Genetic analysis of the E site during RF2 programmed frameshifting

The roles of the ribosomal E site are not fully understood. Prior evidence suggests that deacyl-tRNA in the E site can prevent frameshifting. We hypothesized that if the E-site codon must dissociate from its tRNA to allow for frameshifting, then weak codon:anticodon duplexes should allow for greater frameshifting than stronger duplexes. Using the well-characterized Escherichia coli RF2 (prfB) programmed frameshift to study frameshifting, we mutagenized the E-site triplet to all Unn and Cnn codons. Those variants should represent a very wide range of duplex stability. Duplex stability was estimated using two different methods. Frameshifting is inversely correlated with stability, as estimated by either method. These findings indicate that pairing between the deacyl-tRNA and the E-site codon opposes frameshifting. We discuss the implications of these findings on frame maintenance and on the RF2 programmed frameshift mechanism.

Structures of tRNAs with an expanded anticodon loop in the decoding center of the 30S ribosomal subunit

During translation, some +1 frameshift mRNA sites are decoded by frameshift suppressor tRNAs that contain an extra base in their anticodon loops. Similarly engineered tRNAs have been used to insert nonnatural amino acids into proteins. Here, we report crystal structures of two anticodon stem-loops (ASLs) from tRNAs known to facilitate +1 frameshifting bound to the 30S ribosomal subunit with their cognate mRNAs. ASL(CCCG) and ASL(ACCC) (5'-3' nomenclature) form unpredicted anticodon-codon interactions where the anticodon base 34 at the wobble position contacts either the fourth codon base or the third and fourth codon bases. In addition, we report the structure of ASL(ACGA) bound to the 30S ribosomal subunit with its cognate mRNA. The tRNA containing this ASL was previously shown to be unable to facilitate +1 frameshifting in competition with normal tRNAs (Hohsaka et al. 2001), and interestingly, it displays a normal anticodon-codon interaction. These structures show that the expanded anticodon loop of +1 frameshift promoting tRNAs are flexible enough to adopt conformations that allow three bases of the anticodon to span four bases of the mRNA. Therefore it appears that normal triplet pairing is not an absolute constraint of the decoding center.

Expanding the genetic code

Recently, a general method was developed that makes it possible to genetically encode unnatural amino acids with diverse physical, chemical, or biological properties in Escherichia coli, yeast, and mammalian cells. More than 30 unnatural amino acids have been incorporated into proteins with high fidelity and efficiency by means of a unique codon and corresponding tRNA/aminoacyl-tRNA synthetase pair. These include fluorescent, glycosylated, metal-ion-binding, and redox-active amino acids, as well as amino acids with unique chemical and photochemical reactivity. This methodology provides a powerful tool both for exploring protein structure and function in vitro and in vivo and for generating proteins with new or enhanced properties.

Recognition and positioning of mRNA in the ribosome by tRNAs with expanded anticodons

Mutant tRNAs containing an extra nucleotide in the anticodon loop are known to suppress +1 frameshift mutations, but in no case has the molecular mechanism been clarified. It has been proposed that the expanded anticodon pairs with a complementary mRNA sequence (the frameshift sequence) in the A site, and this quadruplet "codon-anticodon" helix is translocated to the P site to restore the correct reading frame. Here, we analyze the ability of tRNA analogs containing expanded anticodons to recognize and position mRNA in ribosomal complexes in vitro. In all cases tested, 8 nt anticodon loops position the 3' three-quarters of the frameshift sequence in the P site, indicating that the 5' bases of the expanded anticodon (nucleotides 33.5, 34, and 35) pair with mRNA in the P site. We also provide evidence that four base-pairs can form between the P-site tRNA and mRNA, and the fourth base-pair involves nucleotide 36 of the tRNA and lies toward (or in) the 30 S E site. In the A site, tRNA analogs with the expanded anticodon ACCG are able to recognize either CGG or GGU. These data imply a flexibility of the expanded anticodon in the A site. Recognition of the 5' three-quarters of the frameshift sequence in the A site and subsequent translocation of the expanded anticodon to the P site results in movement of mRNA by four nucleotides, explaining how these tRNAs can change the mRNA register in the ribosome to restore the correct reading frame.

Non-bridging phosphate oxygen atoms within the tRNA anticodon stem-loop are essential for ribosomal A site binding and translocation

The conformation of the anticodon stem-loop of tRNAs required for correct decoding by the ribosome depends on intramolecular and intermolecular interactions that are independent of the tRNA nucleotide sequence. Non-bridging phosphate oxygen atoms have been shown to be critical for the structure and function of several RNAs. However, little is known about the role they play in ribosomal A site binding and translocation of tRNA to the P site. Here, we show that non-bridging phosphate oxygen atoms within the tRNA anticodon stem-loop at positions 33, 35, and 37 are important for A site binding. Those at positions 34 and 36 are not necessary for binding, but are essential for translocation. Our results correlate with structural data, indicating that position 34 interacts with the highly conserved 16S rRNA base G966 and position 36 interacts with the universally conserved tRNA base U33 during translocation to the P site.

Expanding the Genetic Code

Although chemists can synthesize virtually any small organic molecule, our ability to rationally manipulate the structures of proteins is quite limited, despite their involvement in virtually every life process. For most proteins, modifications are largely restricted to substitutions among the common 20 amino acids. Herein we describe recent advances that make it possible to add new building blocks to the genetic codes of both prokaryotic and eukaryotic organisms. Over 30 novel amino acids have been genetically encoded in response to unique triplet and quadruplet codons including fluorescent, photoreactive, and redox-active amino acids, glycosylated amino acids, and amino acids with keto, azido, acetylenic, and heavy-atom-containing side chains. By removing the limitations imposed by the existing 20 amino acid code, it should be possible to generate proteins and perhaps entire organisms with new or enhanced properties.

An expanded genetic code with a functional quadruplet codon

With few exceptions the genetic codes of all known organisms encode the same 20 amino acids, yet all that is required to add a new building block are a unique tRNA/aminoacyl-tRNA synthetase pair, a source of the amino acid, and a unique codon that specifies the amino acid. For example, the amber nonsense codon, TAG, together with orthogonal Methanococcus jannaschii or Escherichia coli tRNA/synthetase pairs have been used to genetically encode a variety of unnatural amino acids in E. coli and yeast, respectively. However, the availability of noncoding triplet codons ultimately limits the number of amino acids encoded by any organism. Here, we report the design and generation of an orthogonal synthetase/tRNA pair derived from archaeal tRNA(Lys) sequences that efficiently and selectively incorporates an unnatural amino acid into proteins in response to the quadruplet codon, AGGA. Frameshift suppression with L-homoglutamine (hGln) does not significantly affect protein yields or cell growth rates and is mutually orthogonal with amber suppression, permitting the simultaneous incorporation of two unnatural amino acids, hGln and O-methyl-L-tyrosine, at distinct positions within myoglobin. This work suggests that neither the number of available triplet codons nor the translational machinery itself represents a significant barrier to further expansion of the genetic code.

Fidelity of aminoacyl-tRNA selection on the ribosome: kinetic and structural mechanisms

The ribosome discriminates between correct and incorrect aminoacyl-tRNAs (aa-tRNAs), or their complexes with elongation factor Tu (EF-Tu) and GTP, according to the match between anticodon and mRNA codon in the A site. Selection takes place at two stages, prior to GTP hydrolysis (initial selection) and after GTP hydrolysis but before peptide bond formation (proofreading). In part, discrimination results from different rejection rates that are due to different stabilities of the respective codon-anticodon complexes. An important additional contribution is provided by induced fit, in that only correct codon recognition leads to acceleration of rate-limiting rearrangements that precede chemical steps. Recent elucidation of ribosome structures and mutational analyses suggest which residues of the decoding center may be involved in signaling formation of the correct codon-anticodon duplex to the functional centers of the ribosome. In utilizing induced fit for substrate discrimination, the ribosome resembles other nucleic acid-programmed polymerases.

Exploring the limits of codon and anticodon size

We previously employed a combinatorial approach to identify the most efficient suppressors of four-base codons in E. coli. We have now examined the suppression of two-, three-, four-, five-, and six-base codons with tRNAs containing 6-10 nt in their anticodon loops. We found that the E. coli translational machinery tolerates codons of 3-5 bases and that tRNAs with 6-10 nt anticodon loops can suppress these codons. However, N-length codons were found to prefer N + 4-length anticodon loops. Additionally, sequence preferences, including the requirement of Watson-Crick complementarity to the codon, were evident in the loops. These selections have yielded efficient suppressors of four-base and five-base codons for our ongoing efforts to expand the genetic code. They also highlight some of the parameters that underlie the fidelity of frame maintenance.

Expanding the genetic code: selection of efficient suppressors of four-base codons and identification of "shifty" four-base codons with a library approach in Escherichia coli

Naturally occurring tRNA mutants are known that suppress +1 frameshift mutations by means of an extended anticodon loop, and a few have been used in protein mutagenesis. In an effort to expand the number of possible ways to uniquely and efficiently encode unnatural amino acids, we have devised a general strategy to select tRNAs with the ability to suppress four-base codons from a library of tRNAs with randomized 8 or 9 nt anticodon loops. Our selectants included both known and novel suppressible four-base codons and resulted in a set of very efficient, non-cross-reactive tRNA/four-base codon pairs for AGGA, UAGA, CCCU and CUAG. The most efficient four-base codon suppressors had Watson-Crick complementary anticodons, and the sequences of the anticodon loops outside of the anticodons varied with the anticodon. Additionally, four-base codon reporter libraries were used to identify "shifty" sites at which +1 frameshifting is most favorable in the absence of suppressor tRNAs in Escherichia coli. We intend to use these tRNAs to explore the limits of unnatural polypeptide biosynthesis, both in vitro and eventually in vivo. In addition, this selection strategy is being extended to identify novel five- and six-base codon suppressors.

Yeast frameshift suppressor mutations in the genes coding for transcription factor Mbf1p and ribosomal protein S3: evidence for autoregulation of S3 synthesis

The SUF13 and SUF14 genes were identified among extragenic suppressors of +1 frameshift mutations. SUF13 is synonymous with MBF1, a single-copy nonessential gene coding for a POLII transcription factor. The suf13-1 mutation is a two-nucleotide deletion in the SUF13/MBF1 coding region. A suf13::TRP1 null mutant suppresses +1 frameshift mutations, indicating that suppression is caused by loss of SUF13 function. The suf13-1 suppressor alters sensitivity to aminoglycoside antibiotics and reduces the accumulation of his4-713 mRNA, suggesting that suppression is mediated at the translational level. The SUF14 gene is synonymous with RPS3, a single-copy essential gene that codes for the ribosomal protein S3. The suf14-1 mutation is a missense substitution in the coding region. Increased expression of S3 limits the accumulation of SUF14 mRNA, suggesting that expression is autoregulated. A frameshift mutation in SUF14 that prevents full-length translation eliminated regulation, indicating that S3 is required for regulation. Using CUP1-SUF14 and SUF14-lacZ fusions, run-on transcription assays, and estimates of mRNA half-life, our results show that transcription plays a minor role if any in regulation and that the 5'-UTR is necessary but not sufficient for regulation. A change in mRNA decay rate may be the primary mechanism for regulation.

Decoding of tandem quadruplets by adjacent tRNAs with eight-base anticodon loops

To expand the genetic code for specification of multiple non-natural amino acids, unique codons for these novel amino acids are needed. As part of a study of the potential of quadruplets as codons, the decoding of tandem UAGA quadruplets by an engineered tRNA(Leu) with an eight-base anticodon loop, has been investigated. When GCC is the codon immediately 5' of the first UAGA quadruplet, and release factor 1 is partially inactivated, the tandem UAGAs specify two leucines with an overall efficiency of at least 10%. The presence of a purine at anticodon loop position 32 of the tRNA decoding the codon 5' to the first UAGA seems to influence translation of the following codon. Another finding is intraribosomal dissociation of anticodons from codons and their re-pairing to mRNA at overlapping or nearby codons. In one case where GCC is replaced by CGG, only a single Watson-Crick base pair can form upon re-pairing when decoding is resumed. This has implications for the mechanism of some cases of programmed frameshifting.

Quadruplet codons: implications for code expansion and the specification of translation step size

One of the requirements for engineering expansion of the genetic code is a unique codon which is available for specifying the new amino acid. The potential of the quadruplet UAGA in Escherichia coli to specify a single amino acid residue in the presence of a mutant tRNA(Leu) molecule containing the extra nucleotide, U, at position 33.5 of its anticodon loop has been examined. With this mRNA-tRNA combination and at least partial inactivation of release factor 1, the UAGA quadruplet specifies a leucine residue with an efficiency of 13 to 26 %. The decoding properties of tRNA(Leu) with U at position 33.5 of its eight-membered anticodon loop, and a counterpart with A at position 33.5, strongly suggest that in both cases their anticodon loop bases stack in alternative conformations. The identity of the codon immediately 5' of the UAGA quadruplet influences the efficiency of quadruplet translation via the properties of its cognate tRNA. When there is the potential for the anticodon of this tRNA to dissociate from pairing with its codon and to re-pair to mRNA at a nearby 3' closely matched codon, the efficiency of quadruplet translation at UAGA is reduced. Evidence is presented which suggests that when there is a purine base at position 32 of this 5' flanking tRNA, it influences decoding of the UAGA quadruplet.

Translational frameshifting: implications for the mechanism of translational frame maintenance

The ribosome rapidly translates the information in the nucleic sequence of mRNA into the amino acid sequence of proteins. As with any biological process, translation is not completely accurate; it must compromise the antagonistic demands of increased speed and greater accuracy. Yet, reading-frame errors are especially infrequent, occurring at least 10 times less frequently than other errors. How do ribosomes maintain the reading frame so faithfully? Geneticists have addressed this question by identifying suppressors that increase error frequency. Most familiar are the frameshift suppressor tRNAs, though other suppressors include mutant forms of rRNA, ribosomal proteins, or translation factors. Certain mRNA sequences can also program frameshifting by normal ribosomes. The models of suppression and programmed frameshifting describe apparently quite different mechanisms. Contemporary work has questioned the long-accepted model for frameshift suppression by mutant tRNAs, and a unified explanation has been proposed for both phenomena. The Quadruplet Translocation Model proposes that suppressor tRNAs cause frameshifting by recognizing an expanded mRNA codon. The new data are inconsistent with this model for some tRNAs, implying the model may be invalid for all. A new model for frameshift suppression involves slippage caused by a weak, near-cognate codon.anticodon interaction. This strongly resembles the mechanism of +1 programmed frameshifting. This may mean that infrequent frameshift errors by normal ribosomes may result from two successive errors: misreading by a near-cognate tRNA, which causes a subsequent shift in reading frame. Ribosomes may avoid phenotypically serious frame errors by restricting apparently innocuous errors of sense.

tRNA imbalance promotes -1 frameshifting via near-cognate decoding

tRNAGly1 is the Escherichia coli glycine tRNA specific for GGG codons. A genetic selection for multicopy suppressors of a frameshift mutation has shown that increased levels of wild-type tRNAGly1 causes -1 frameshifting. Analysis of the suppression spectrum of this multicopy suppressor and peptide sequencing of the suppressed protein product showed that it promoted GG doublet decoding at the near-cognate GGA codons. It is proposed that increasing the concentration of the GGG-specific tRNAGly1 relative to the cognate GGA-decoding tRNAGly2 allows the near-cognate tRNA to read GGA codons. Near-cognate decoding of GGA codons by tRNAGly1 can occur by a two-out-of-three reading mechanism, in which only the first two bases of the GGA codon are paired with the anticodon, thus permitting doublet translocations. In mycoplasmas, a single tRNA typically decodes all four triplets of a codon family and introduction of a feature of the Mypoplasma mycoides tRNAGly responsible for non-discriminate decoding, a C at position 32, into the anticodon E. coli tRNAGly1, enhanced the efficiency of doublet decoding.

A new model for phenotypic suppression of frameshift mutations by mutant tRNAs

According to the prevailing model, frameshift-suppressing tRNAs with an extra nucleotide in the anticodon loop suppress +1 frameshift mutations by recognizing a four-base codon and promoting quadruplet translocation. We present three sets of experiments that suggest a general alternative to this model. First, base modification should actually block such a four-base interaction by two classical frameshift suppressors. Second, for one Salmonella suppressor tRNA, it is not mutant tRNA but a structurally normal near cognate that causes the +1 shift in-frame. Finally, frameshifting occurs in competition with normal decoding of the next in-frame codon, consistent with an event that occurs in the ribosomal P site after the translocation step. These results suggest an alternative model involving peptidyl-tRNA slippage at the classical CCC-N and GGG-N frameshift suppression sites.

Analysis of interactions between the codon-anticodon duplexes within the ribosome: their role in translation

Computer graphics simulation of interactions between the codon-anticodon duplexes formed by normal elongator tRNAs at the ribosomal A, P and E-sites (the AP and PE interduplex interactions) was made. This demonstrated that only the correct duplexes at the A-site are compatible with the AP interduplex interaction. The selection of synonymous codons and anticodon wobble bases, together with the AP interduplex interaction, prevents frameshifting. In the absence of this interaction the efficiency of the selection falls off sharply. This suggests that the AP interduplex interaction should be retained during translocation and in the post-translocation state, i.e. the PE interduplex interaction that is identical with that of AP should exist to avoid frameshifting. In such a model the P-site duplex provides an indirect linkage between the A and E-site duplexes. The indirect linkage prohibits the simultaneous existence of the A and E-site duplexes. The wobble pairs of the P and E-site duplexes can affect the rate of the A-site occupation via the AP interduplex interaction and the AE interduplex indirect linkage. It is demonstrated that frameshifting can occur from the AP or PE codon-anticodon complex destabilization caused, for example, by small mobility of the wobble pairs, misreading of the codon, unmodified adenine and guanine at tRNA positions 34 (wobble) and 37, respectively. The results obtained can be subjected to direct experimental tests.

Selection of aminoacyl-tRNAs at sense codons: the size of the tRNA variable loop determines whether the immediate 3' nucleotide to the codon has a context effect

Codon context can affect translational efficiency by several molecular mechanisms. The base stacking interactions between a codon-anticodon complex and the neighboring nucleotide immediately 3' can facilitate translation by amber suppressors and the tRNA structure is also known to modulate the sensitivity to context. In this study the relative rates of aminoacyl-tRNA selection were measured at four sense codons (UGG, CUC, UUC and UCA), in all four 3' nucleotide contexts, through direct competition with a programmed frameshift at a site derived from the release factor 2 gene. Two codons (UGG and UUC) are read by tRNAs with small variable regions and their rates of aminoacyl-tRNA selection correlated with the potential base stacking strength of the 3' neighboring nucleotide. The other two codons (CUC and UCA) are read by tRNAs with large variable regions and the rate of selection of the aminoacyl-tRNAs in these cases varied little among the four contexts. Re-examination of published data on amber suppression also revealed an inverse correlation between context sensitivity and the size of the variable region. Collectively the data suggest that a large variable loop in a tRNA decreases the influence of the 3' context on tRNA selection, probably by strengthening tRNA-ribosomal interactions.

Pulling the ribosome out of frame by +1 at a programmed frameshift site by cognate binding of aminoacyl-tRNA

Programmed translational frameshifts efficiently alter a translational reading frame by shifting the reading frame during translation. A +1 frameshift has two simultaneous requirements: a translational pause which occurs when either an inefficiently recognized sense or termination codon occupies the A site, and the presence of a special peptidyl-tRNA occupying the P site during the pause. The special nature of the peptidyl-tRNA reflects its ability to slip +1 on the mRNA or to facilitate binding of an incoming aminoacyl-tRNA out of frame in the A site. This second mechanism suggested that in some cases the first +1 frame tRNA could have an active role in frameshifting. We found that overproducing this tRNA can drive frameshifting, surprisingly regardless of whether frameshifting occurs by peptidyl-tRNA slippage or out-of-frame binding of aminoacyl-tRNA. This finding suggests that in both cases, the shift in reading frame occurs coincident with formation of a cognate codon-anticodon interaction in the shifted frame.

Special peptidyl-tRNA molecules can promote translational frameshifting without slippage

Recently we described an unusual programmed +1 frameshift event in yeast retrotransposon Ty3. Frameshifting depends on the presence of peptidyl-tRNA(AlaCGC) on the GCG codon in the ribosomal P site and on a translational pause stimulated by the slowly decoded AGU codon. Frameshifting occurs on the sequence GCG-AGU-U by out-of-frame binding of a valyl-tRNA to GUU without slippage of peptidyl-tRNA(AlaCGC). This mechanism challenges the conventional understanding that frameshift efficiency must correlate with the ability of mRNA-bound tRNA to slip between cognate or near-cognate codons. Though frameshifting does not require slippery tRNAs, it does require special peptidyl-tRNAs. We show that overproducing a second isoacceptor whose anticodon had been changed to CGC eliminated frameshifting; peptidyl-tRNA(AlaCGC) must have a special capacity to induce +1 frameshifting in the adjacent ribosomal A site. In order to identify other special peptidyl-tRNAs, we tested the ability of each of the other 63 codons to replace GCG in the P site. We found no correlation between the ability to stimulate +1 frameshifting and the ability of the cognate tRNA to slip on the mRNA--several codons predicted to slip efficiently do not stimulate frameshifting, while several predicted not to slip do stimulate frameshifting. By inducing a severe translational pause, we identified eight tRNAs capable of inducing measurable +1 frameshifting, only four of which are predicted to slip on the mRNA. We conclude that in Saccharomyces cerevisiae, special peptidyl-tRNAs can induce frameshifting dependent on some characteristic(s) other than the ability to slip on the mRNA.

Special peptidyl-tRNA molecules can promote translational frameshifting without slippage

Recently we described an unusual programmed +1 frameshift event in yeast retrotransposon Ty3. Frameshifting depends on the presence of peptidyl-tRNA(AlaCGC) on the GCG codon in the ribosomal P site and on a translational pause stimulated by the slowly decoded AGU codon. Frameshifting occurs on the sequence GCG-AGU-U by out-of-frame binding of a valyl-tRNA to GUU without slippage of peptidyl-tRNA(AlaCGC). This mechanism challenges the conventional understanding that frameshift efficiency must correlate with the ability of mRNA-bound tRNA to slip between cognate or near-cognate codons. Though frameshifting does not require slippery tRNAs, it does require special peptidyl-tRNAs. We show that overproducing a second isoacceptor whose anticodon had been changed to CGC eliminated frameshifting; peptidyl-tRNA(AlaCGC) must have a special capacity to induce +1 frameshifting in the adjacent ribosomal A site. In order to identify other special peptidyl-tRNAs, we tested the ability of each of the other 63 codons to replace GCG in the P site. We found no correlation between the ability to stimulate +1 frameshifting and the ability of the cognate tRNA to slip on the mRNA--several codons predicted to slip efficiently do not stimulate frameshifting, while several predicted not to slip do stimulate frameshifting. By inducing a severe translational pause, we identified eight tRNAs capable of inducing measurable +1 frameshifting, only four of which are predicted to slip on the mRNA. We conclude that in Saccharomyces cerevisiae, special peptidyl-tRNAs can induce frameshifting dependent on some characteristic(s) other than the ability to slip on the mRNA.

Evidence that GHN phase bias does not constitute a framing code

It has been suggested that a triplet repeated pattern found in coding sequences, the G-nonG-N or GHN phase bias, serves a framing function during protein synthesis. To test this idea, the framing characteristics of a highly GHN biased sequence are examined. No effects on reading frame maintenance are observed despite the use of sensitive frameshift assays. Specifically, first the GHN phase is not more accurate than the alternative overlapping phases (i.e., HNG and NGH). Second, ribosomes do not exhibit any significant tendency to slip from the alternative frames into the GHN pahse. In addition, examination of Escherichia coli programmed frameshift sites does not support roles for GHN phase bias in programmed frameshifting. Framing functions for GHN phase bias, if they occur at all, must be extremely limited.

A novel programed frameshift expresses the POL3 gene of retrotransposon Ty3 of yeast: frameshifting without tRNA slippage

Most retroviruses and retrotransposons express their pol gene as a translational fusion to the upstream gag gene, often involving translational frameshifting. We describe here an unusual translational frameshift event occurring between the GAG3 and POL3 genes of the retrotransposon Ty3 of yeast. A +1 frameshift occurs within the sequence GCG AGU U (shown as codons of GAG3), encoding alanine-valine (GCG A GUU). Unlike other programed translational frameshifts described, this event does not require tRNA slippage between cognate or near-cognate codons in the mRNA. Two features distal to the GCG codon stimulate frameshifting. The low availability of the tRNA specific for the "hungry" serine codon, AGU, induces a translational pause required for frameshifting. A sequence of 12 nt distal to the AGU codon (termed the Ty3 "context") also stimulates the event.

Analysis of effects of tRNA:message stability on frameshift frequency at the Escherichia coli RF2 programmed frameshift site

The codon that is in-frame prior to +1 frameshifting at the E.coli prfB (RF2 gene) frameshift site is randomized to create thirty-two variants. These alleles vary 1000-fold in frameshift-dependent expression in fusions to lacZ. Frameshifting is more frequent at sites where the in-frame codon ends in uridine, as if third position wobble pairs to message uridine facilitate slippage into the +1 frame. Consistent with other studies of programmed frameshift sites, efficient frameshifting depends on stable message:tRNA base pairs after rephasing. For complexes with mispairs, frameshift frequency depends on the nature, number, and position of mispairs. Central purine:purine mispairs are especially inhibitory. Relative stabilities of +1 rephased complexes are estimated from published data on the stabilities of tRNA:tRNA complexes. Stability correlates with frameshifting over its entire range, which suggests that stability is an important determinant of the probability of translation of the rephased complex.

Involvement of the size and sequence of the anticodon loop in tRNA recognition by mammalian and E. coli methionyl-tRNA synthetases

The rates of the cross-aminoacylation reactions of tRNAs(Met) catalyzed by methionyl-tRNA synthetases from various organisms suggest the occurrence of two types of tRNA(Met)/methionyl-tRNA synthetase systems. In this study, the tRNA determinants recognized by mammalian or E. coli methionyl-tRNA synthetases, which are representative members of the two types, have been examined. Like its prokaryotic counterpart, the mammalian enzyme utilizes the anticodon of tRNA as main recognition element. However, the mammalian cytoplasmic elongator tRNA(Met) species is not recognized by the bacterial synthetase, and both the initiator and elongator E. coli tRNA(Met) behave as poor substrates of the mammalian cytoplasmic synthetase. Synthetic genes encoding variants of tRNAs(Met), including the elongator one from mammals, were expressed in E. coli. tRNAs(Met) recognized by a synthetase of a given type can be converted into a substrate of an enzyme of the other type by introducing one-base substitutions in the anticodon loop or stem. In particular, a reduction of the size of the anticodon loop of cytoplasmic mammalian elongator tRNA(Met) from 9 to 7 bases, through the creation of an additional Watson-Crick pair at the bottom of the anticodon stem, makes it a substrate of the prokaryotic enzyme and decreases its ability to be methionylated by the mammalian enzyme. Moreover, enlarging the size of the anticodon loop of E. coli tRNA(Metm) from 7 to 9 bases, by disrupting the base pair at the bottom of the anticodon stem, renders the resulting tRNA a good substrate of the mammalian enzyme, while strongly altering its reaction with the prokaryotic synthetase. Finally, E. coli tRNA(Metf) can be rendered a better substrate of the mammalian enzyme by changing its U33 into a C. This modification makes the sequence of the anticodon loop of tRNA(Metf) identical to that of cytoplasmic initiator tRNA(Met).

Effects of the nucleotide 3' to an amber codon on ribosomal selection rates of suppressor tRNA and release factor-1

Rates of ribosomal selection of both release factor 1 (RF1) and a suppressor tRNA (Su7C33) were studied at an amber codon at which the 3' neighbor was permuted. Rates of RF1 selection vary 2.6-fold among contexts. The 3' neighbor-dependent variation of RF1 action correlates very strongly with the non-random frequencies of 3' neighbors at UAG terminators (r = 0.97), which argues that the rate of RF1 selection is an important determinant 3' neighbor choice at termination codons. The data are consistent with a model for RF1 selection in which RF1 makes a specific contact(s) to the 3' neighbor and that this interaction is most favorable to uridylic acid. Measured rates of Su7C33 selection vary fivefold among 3' contexts. We also develop a method to calculate rates of selection for other suppressors, based on the assumption that rates of RF1 selection at each 3' context can be generalized to other sites that have the same 3' neighbor. Rates for various suppressors appear to vary from two- to fivefold depending on the 3' neighbor. Generally, the rate of selection of suppressors at different contexts correlates with the stacking strength of the 3' neighbor as measured in vitro. The two- to fivefold range of 3' neighbor effects on rate of aminoacyl-tRNA selection is greater than that previously observed within sets of codons read by the same tRNA. It is suggested that the choice of codons to achieve favorable contexts may be more important than the choice of a common codon at some message sites.