THIOBACILLUS-FERROOXIDANS TYROSYL-TRANSFER-RNA SYNTHETASE FUNCTIONS IN-VIVO IN ESCHERICHIA-COLI

The tyrosyl-tRNA synthetase gene (tyrZ) from Thiobacillus ferrooxidans , an acidophilic, autotrophic, gram-negative bacterium that participat es in bioleaching of minerals, was cloned and sequenced. The encoded p olypeptide (TyrRZ) is 407 amino acids in length (molecular mass; 38 kD a). The predicted protein sequence has an extensive overall identity ( 44%) to the sequence of the protein encoded by the Bacillus subtilis t yrZ gene, one of the two genes encoding tyrosyl-tRNA synthetases in th is microorganism. Alignment with Escherichia coli TyrRS revealed limit ed overall identity (24%), except in the regions of the signature sequ ence for class I aminoacyl-tRNA synthetases. Complementation of an E. coli strain with a thermosensitive mutation in TyrRS showed that the p rotein encoded by the T.ferrooxidans tyrZ gene is functional and recog nizes the E. coli tRNA(Tyr) as a substrate. TyrZ is a single-copy gene as revealed by Southern blot analysis. The gene was localized upstrea m from the putative promoters of the rrnT2 ribosomal RNA operon. Altho ugh no rho-independent transcription terminator was found between the two genes, a 1.3-kb RNA hybridized to a DNA probe derived from the tyr Z gene. The functional relationship between these two transcription un its is discussed.