Histidine regulatory mutants in Salmonella typhimurium. II. Histidine regulatory mutants having altered histidyl-tRNA synthetase

1. (1) The hisS gene of Salmonella typhimurium is the structural gene for histidyl-tRNA synthetase. Mutants in the hisS gene have an altered histidyl-tRNA synthetase and constitute one of the four classes of histidine regulatory mutants. 2. (2) Mutant hisS1520 has a synthetase with a 50-fold decreased affinity for histidine. This mutant has a markedly decreased growth rate on minimal medium and is de-repressed for the biosynthetic enzymes of the histidine operon. Both of these properties appear to reflect the decreased amount of charged histidine-specific tRNA due to the mutant synthetase. The amount of histidyl-tRNA can be increased in this mutant by swelling its internal pool of histidine, and this swelling results in restoration of the normal growth rate and repression of the operon. These results support the idea that histidyl-tRNA is more directly connected with the repression of the histidine operon than histidine itself. 3. (3) Levels of the histidyl-tRNA synthetase are unaffected by de-repression of the histidine operon.