Abstract
A 4459 bp long BamHI restriction fragment containing the two genes for the Thermus thermophilus HB8 phenylalanyl-tRNA synthetase was cloned in Escherichia coli and its nucleotide sequence was determined. The genes pheS and pheT encode the alpha- and beta-subunits with a molecular weight of 39 and 87 kD, respectively. Three conserved sequence motifs typical for class II tRNA synthetases occur in the alpha-subunit. Secondary structure predictions indicate that an arm composed of two anti-parallel alpha-helices similar to that reported for the E.coli seryl-tRNA synthetase may be present in its N-terminal portion. In the beta-subunit clusters of hydrophilic amino acids and a leucine zipper motif were identified, and several pronounced alpha-helical regions were predicted. The particular arginine and lysine residues in the N-terminal portion of the beta-subunit, which were found to participate in tRNA binding in the yeast and E.coli PheRSs, have their counterparts in the T.thermophilus protein. The 5'-portion of an open reading frame downstream of pheT was found and codes for a yet unidentified, extremely hydrophobic peptide. The pheST genes are presumably cotranscribed and translationally coupled. A novel type of a putative transcriptional terminator in Thermus species was identified immediately downstream of pheT and other Thermus genes. The genes pheS and pheST were expressed in E.coli.