Catalytic defects in mutants of class II histidyl-tRNA synthetase from Salmonella typhimurium previously linked to decreased control of histidine biosynthesis regulation

The expression of histidine biosynthetic genes in enteric bacteria is regulated by an attenuation mechanism in which the level of histidyl-tRNA serves as a key sensor of the intracellular histidine pool. Among the early observations that led to the formation of this model for Salmonella typhimurium were the identification of mutants in the gene (hisS) encoding histidyl-tRNA synthetase. We report here the detailed biochemical characterization of five of these S. typhimurium bradytrophic mutants isolated by selection for resistance to histidine analogs, including identification of the deduced amino acid substitutions and determination of the resulting effects on the kinetics of adenylation and aminoacylation. Using the crystal structure of the closely related Escherichia coli histidyl-tRNA synthetase (HisRS) as a guide, two mutants were mapped to a highly conserved proline residue in motif 2 (P117S, P117Q), and were correlated with a fivefold decrease in the k(cat) for the pyrophosphate exchange reaction, as well as a tenfold increase in the K-m for tRNA in the aminoacylation reaction. Another mutant substitution (A302T) mapped to a residue adjacent to the histidine binding pocket, leading to a tenfold increase in K-m for histidine in the pyrophosphate exchange reaction. The remaining two mutants (S167F, N254T) substitute residues in or directly adjacent to the hinge region, which joins the insertion domain between motif 2 and motif 3 to the catalytic core, and cause the K-m for tRNA to increase four- to tenfold. The kinetic analysis of these mutants establishes a direct link between critical interactions within the active site of HisRS and regulation of histidine biosynthesis, and provides further evidence for the importance of local conformational changes during the catalytic cycle. (C) 1998 Academic Press.