SPECIES-SPECIFIC MICROHELIX AMINOACYLATION BY A EUKARYOTIC PATHOGEN TRANSFER-RNA SYNTHETASE DEPENDENT ON A SINGLE-BASE PAIR

We report here that tyrosyl-tRNA synthetase from the eukaryotic pathog en Pneumocystis canmz is a 370 ainino acid polypeptide with characteri stic elements of a class I aminoacyl-tRNA synthetase and aligns with t he prokaryotic tyrosyl-tRNA synthetases in the class-defining active s ite region, including the tRNA acceptor helix-binding region. The expr essed enzyme is a dimer that aminoacylates yeast tRNA but not Escheric hia coli tRNA(Tyr). Like most tRNAs, prokaryotic tyrosine tRNAs have a G1 . C72 basel pair at the ends of their respective acceptor helices. However, the eukaryote cytoplasmic tyrosine tRNAs have an uncommon C1 . G72 base pair. We show that P. carinii tyrosyl-tRNA synthetase char ges a seven base pair hairpin microhelix (microhelix(Tyr)) whose seque nce is derived from the acceptor stem of yeast cytoplasmic tRNA(Tyr). In contrast, the enzyme does not charge E. coli microhelix(Tyr). Chang ing the C1 . G72 of yeast microhelix(Tyr) to G1 . C72 abolishes chargi ng by the P. carinii tyrosyl-tRNA synthetase. Conversely, we found tha t E. coli tyrosyl-tRNA synthetase can charge an E. coli microhelix(Tyr ) and that charging is sensitive to having a G1 . C72 rather than a C1 . G72 base pair. The results demonstrate that the common structural f ramework of homologous tRNA synthetases has the capacity to coadapt to a transversion in a critical acceptor helix base pair and that this c oadaptation can account for species-selective microhelix aminoacylatio n. We propose that species-selective acceptor helix recognition can be used as a conceptual basis for species-specific inhibitors of tRNA sy nthetases.