Life without the essential bacterial tRNA Ile2-lysidine synthetase TilS: a case of tRNA gene recruitment in Bacillus subtilis.

In eubacteria, the post-transcriptional modification of the wobble cytidine of the CAU anticodon in a precursor tRNA(Ile2) to a lysidine residue (2-lysyl-cytidine, abbreviated as L) allows the amino acid specificity to change from methionine to isoleucine and the codon decoding specificity to shift from AUG to AUA. The tilS gene encoding the enzyme that catalyses this modification is widely distributed. However, some microbial species lack a tilS gene, indicating that an alternative strategy exists to accurately translate the AUA codon into Ile. To determine whether a TilS-dependent bacterium, such as Bacillus subtilis, can overcome the absence of lysidine in its tRNA(Ile2) (CAU), we analysed the suppressor mutants of a tilS-thermosensitive allele. These tilS-suppressor mutants carry a substitution of the wobble guanosine into thymidine in one of the tRNA(Ile1) genes (the original GAT anticodon is changed to a TAT). In absence of TilS activity, the AUA codons are translated into isoleucine by the suppressor tRNA(Ile1), although a low level of AUA codons is also mistranslated into methionine. Results are in agreement with rare cases of eubacteria (and archaea), which naturally lack the tilS gene (or tiaS in archaea) but contain a tRNA(Ile2) gene containing a TAT instead of a CAT anticodon.