Regulation of histidine operon does not require hisG enzyme

Mutations are described which delete all or part of the first structural gene (hisG) of the histidine operon of Salmonella typhimurium. Physiological regulation of histidine enzymes occurs normally in strains carrying any deletion that has both endpoints within the hisG gene. Constitutive high operon expression is observed in strains carrying any hisG deletion and an unlinked regulatory mutation, hisT1504. These results strongly indicate that the hisG protein is not an essential component of the mechanism for regulating expression of the histidine operon.