Protein synthesis is a highly regulated, three-stage process, comprised of initiation, elongation and termination. In brief, during initiation, the messenger RNA (mRNA) binds to the mRNA channel on the small ribosomal subunit and the start-codon is positioned within the P-(peptidyl) decoding site. A charged initiator transfer RNA (tRNA), which in prokaryotes is normally (formyl) methionyl-tRNAi (met-tRNAfMet) and in eukaryotes is tRNAiMet, is recruited to the start-codon. Importantly, by decoding the first triplet of the mRNA, the initiator tRNA determines the reading frame. The large ribosomal subunit joins to form a ribosomal complex with the initiator tRNA in the P-site of the ribosome, which is primed for the elongation stage, where the message is decoded. During the elongation process, an aminoacyl-tRNA that has an anticodon complementary to the mRNA codon is recruited to the ribosomal A-site. A peptide bond is formed by transfer of the amino acid/peptide attached to the tRNA in the P-site to the aminoacyl-tRNA in the adjacent A-site, and the newly formed peptidyl-tRNA is subsequently translocated from the A-site to the P-site, in conjunction with the mRNA. These processes are promoted by both the ribosome and elongation factors. When a termination codon enters the A-site, termination factors bind to the ribosome and promote the hydrolysis of the peptidyl-tRNA.1
The process of mRNA translation is highly regulated via altering the bioavailability and/or function of its components.1 Although tRNAs were previously considered to have a fairly “passive” role in the overall regulation of protein synthesis, the recent data suggest that, through their modification and bioavailability, they make a major contribution to the global regulation of mRNA translation in all domains of life. With hindsight, this is perhaps unsurprising, since tRNAs play a central role in protein synthesis by providing the link between mRNAs and amino acids. Amino acids are attached to tRNAs through aminoacylation, which is catalysed by aminoacyl-tRNA synthetases. The high level of precision with which the processes of aminoacylation and decoding occur is, in part, achieved through post-transcriptional modification of tRNAs, with over 100 different post-transcriptional modifications described.2 Many of the modifications required to maintain accuracy during decoding target position 34, the wobble nucleotide, and position 37, the nucleotide 3'-adjacent to the anticodon.3 Some modifications, such as methylation resulting in N1-methylguanosine at position 37 (m1G37), are conserved amongst all three domains of life.4 Interestingly, while the majority of tRNA modifications are constitutive, some are dependent on cell state and are influenced by growth or exposure to stress. For example, the tRNA modification enzyme glucose-inhibited division protein (GidA), which catalyses the addition of a carboxymethylaminomethyl group to position 5 of the anticodon wobble uridine in conjunction with the GTPase MnmE,5 is important for the survival of Streptococcus mutans following cell stress.6
