Histidine regulatory mutants in Salmonella typhimurium. VI. Dominance studies

Regulation of the histidine operon in Salmonella typhimurium has been shown to involve at least six separate genes (Roth, Antón & Hartman, 1966; Antón, 1968). Mutations in any one of these separate genes can destroy the ability of the cell to repress fully the histidine biosynthetic enzymes. In order to understand this regulatory mechanism and the nature of these mutations, we have studied the dominance relationships between these mutations and their wild-type alleles. Studies have been done using two F′ episomes derived from Escherichia coli and one derived from Salmonella typhimurium. Four mutant regulatory genes (hisR, hisU, hisW, hisT) are recessive to their wild-type alleles. The hisO mutation is dominant to its wild-type allele but affects only genes located cis to the hisO mutation.