Polar and Antipolar Mutants in the Tryptophan Operon of Salmonella typhimurium

Abstract

Polar mutations in trpA , the first structural gene of the tryptophan operon of Salmonella typhimurium , have an uncoordinate effect on the expression of the distal genes, with trpB , the second gene, being more drastically affected than the last three. A number of these polar mutant strains grow very poorly on anthranilic acid-supplemented minimal medium. By selecting for more rapid growth in the presence of anthranilic acid, secondary mutant clones showing a correction of the polar effect were isolated. A few of these were analyzed and shown to contain deletions of various segments of the trpA gene. Ten randomly isolated deletion mutants missing various segments of the trp operon were analyzed for possible pleiotropic effects. Five of them showed a pleiotropic effect of some sort and five did not. Of those showing pleiotropic effects, one had lost the promotor-like elements necessary to initiate expression of the operon, three showed possible antipolar effects, and one showed both polar and antipolar effects simultaneously.