Decoding system for the AUA codon by tRNA~(lle) with the UAU anticodon in Mycoplasma mobile

摘要

Deciphering the genetic code is a fundamental process in all living organisms. In many bacteria, AUA codons are deciphered by tRNAlle2 bearing lysidine (L) at the wobble position. L is a modified cytidine introduced post-transcriptionally by tRNA,,e-lysidine synthetase (TilS). Some bacteria, including Mycoplasma mobile, do not carry the tilS gene, indicating that they have established a different system to decode AUA codons. In this study, tRNAlle2 has been isolated from M. mobile and was found to contain a UAU anticodon without any modification. Mycoplasma mobile isoleucyl-tRNA synthetase (lleRS) recognized the UAU anticodon, whereas Escherichia coli lleRS did not efficiently aminoacylate tRNAlle2UAU. In M. mobile lleRS, a single Arg residue at position 865 was critical for specificity for the UAU anticodon and, when the corresponding site (W905) in E. coli lleRS was substituted with Arg, the W905R mutant efficiently aminoacylated tRNA with UAU anticodon. Mycoplasma mobile tRNAl,e2 cannot distinguish between AUA and AUG codon on E. coli ribosome. However, on M. mobile ribosome, M. mobile tRNAl,e2UAU specifically recognized AUA codon, and not AUG codon, suggesting M. mobile ribosome has a property that prevents misreading of AUG codon. These findings provide an insight into the evolutionary reorganization of the AUA decoding system.