Genetic analysis of the histidine operon control region of Salmonella typhimurium.

Mutants of the histidine operon control region (hisO) include two classes: (1) those completely unable to express the operon (His auxotrophs), and (2) prototrophs that are unable to achieve fully induced levels of operon expression (still His+ but sensitive to the drug amino-triazole). Using new, as well as previously existing hisO mutants, we constructed a fine-structure deletion map of GO. Mutations that presumably alter the his promoter map at one end of his01 mutations that alter the his attenuator map at the other end of hisO. Between the promoter and the attenuator lie a number of mutations that affect either the translation of the his leader peptide gene, or the formation and stability of his leader messenger RNA structures. All of the point mutations mapping in this central region revert to His+ at a very high frequency (lo-’ to 10m6); this frequency is incremed by both base substitution and frameshift-inducing mutagens. Many of the Hismutants are suppressed by informational suppressors; all three types of nonsense mutations have been identified, demonstrating that translation of a region of his0 between the promoter and attenuator is essential for his operon expression. All of the his0 mutations tested are &-dominant.