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

(1) The hid’ gene of Salmonelb ty(phinzurium is the structural gene for histidyltRNA synthetase. Mutants in the hid gene have an altered histidyl-tRNA synthetase and constitute one of the four classes of histidine regulatory mutants. (2) Mutant hisS1520 has a synthetase with a 60.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 hi&dine operon. Both of these properties appear to reflect the decreased amount of charged histidinespecific tRNA due to the mutant synthetase. The amount of histidyl-tRNA oan be increased in this mutant by swelling its internal pool of hi&dine, 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 con. netted with the repression of the histidine operon than histidine itself. (3) Levels of the histidyl-tRNA synthetase are unaffected by de-repression of the hi&dine operon.