SPECIES-SPECIFIC TRANSFER-RNA RECOGNITION IN RELATION TO TRANSFER-RNASYNTHETASE CONTACT RESIDUES

In spite of variations in the sequences of tRNAs, the genetic code (an ticodon trinucleotides) is conserved in evolution. However, non-antico don nucleotides which are species specific are known to prevent a give n tRNA from functioning in all organisms. Conversely, species-specific tRNA contact residues in synthetases should also prevent cross-specie s acylation in a predictable way. To address this question, we investi gated the relatively small tyrosine tRNA synthetase where contacts of Escherichia coli tRNA(Tyr) with the alpha(2) dimeric protein have been localized by others to four specific sequence clusters on the three-d imensional structure of the Bacillus stearothermophilus enzyme. We use d specific functional tests with a previously not-sequenced and not-ch aracterized Mycobacterium tuberculosis enzyme and showed that it demon strates species-specific aminoacylation in vivo and in vitro. The spec ificity observed fits exactly with the presence of the clusters charac teristic of those established as important for recognition of E. coli tRNA. Conversely, we noted that a recent analysis of the tyrosine enzy me from the eukaryote pathogen Pneumocystis carinii showed just the op posite species specificity of tRNA recognition. According to our align ments, the sequences of the clusters diverge substantially from those seen with the M. tuberculosis, B. stearothermophilus and other enzymes . Thus, the presence or absence of species-specific residues in tRNA s ynthetases correlates in both directions with cross-species aminoacyla tion phenotypes, without reference to the associated tRNA sequences. W e suggest that this kind of analysis can identify those synthetase-tRN A covariations which are needed to preserve the genetic code. These co -variations might be exploited to develop novel antibiotics against pa thogens such as M. tuberculosis and P. carinii. (C) 1997 Academic Pres s Limited.