Thiobacillus ferrooxidans tyrosyl-tRNA synthetase functions in vivo in Escherichia coli

The tyrosyl-tRNA synthetase gene (tyrZ) from Thiobacillus ferrooxidans, an acidophilic, autotrophic, gram-negative bacterium that participates in bioleaching of minerals, was cloned and sequenced. The encoded polypeptide (TyrRZ) is 407 amino acids in length (molecular mass; 38 kDa). The predicted protein sequence has an extensive overall identity (44%) to the sequence of the protein encoded by the Bacillus subtilus tyrZ gene, one of the two genes encoding tyrosyl-tRNA synthetases in this microorganism. Alignment with Escherichia coli TyrRS revealed limited overall identity (24%), except in the regions of the signature sequence for class I aminoacyl-tRNA synthetases. Complementation of an E. coli strain with a thermosensitive mutation in TyrRS showed that the protein encoded by the T. ferrooxidans tyrZ gene is functional and recognizes the E. coli tRNA(Tyr) as a substrate. TyrZ is a single-copy gene as revealed by Southern blot analysis. The gene was localized upstream from the putative promoters of the rrnT2 ribosomal RNA operon. Although no rho-independent transcription terminator was found between the two genes, a 1.3-kb RNA hybridized to a DNA probe derived from the tyrZ gene. The functional relationship between these two transcription units is discussed.