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Effect of Gene Induction on the Rate of Mutagenesis by ICR-191 in Escherichia coli

ICR-191, an acridine half-mustard known to cause frameshift mutations in bacteria, was used to induce Lac− mutations revertible by ICR-191. The reversion rates of several of these mutations were stimulated approximately twofold by the presence of lac inducer. The stimulatory effect of inducer was attributable to gene induction rather than some other effect of inducer, since inducer did not stimulate reversion in a regulator constitutive strain. The stimulatory effect was not observed unless the gene to be reverted was induced during the period of exposure to ICR-191. The presence of a strong polar (nonsense) mutation on the operator side of a frameshift mutation abolished the stimulatory effect of inducer on reversion of the frameshift mutation by ICR-191. (As expected, a nonpolar mutation on the operator side of the frameshift mutation did not affect inducer-stimulated reversion.) It was concluded that some aspect of transcription or translation, or both, in the neighborhood of the ICR-191-induced mutation stimulated reversion by ICR-191. A recA mutation had no effect on reversion by ICR-191 in the presence or absence of inducer. In one mutant, gene induction depressed reversion by ICR-191 about sevenfold. The difference between this exceptional strain and other mutants was not attributable to different genetic backgrounds but seemed to be an inherent difference in the original Lac− mutations.