Frameshift mutants in the histidine operon of Salmonella typhimurium

Two classes of frameshift mutations, (+) types and (−) types, have been isolated in the C gene of the histidine operon of Salmonella typhimurium. All of the (+) type and (−) type mutants are polar, i.e. they have lost C gene function and have reduced amounts of the enzymes corresponding to the genes beyond the C gene in the histidine operon. In nonsense suppressor-free strains, prototrophic double mutants carrying two frameshift mutations of opposite sign, (+ −) types, are all non-polar. In a nonsense suppressor-carrying strain, several polar prototrophic revertants of a frameshift mutation have been isolated. These polar strains become auxotrophic when the nonsense suppressor is removed. As discussed in the paper, all of these results were predicted by the hypothesis (Whitfield, Jr., Martin & Ames, 1966) that frameshift mutants are polar because they generate nonsense codons.