Mutagenic repair in Escherichia coli: VIII. Effect of gyrB mutations on ultravioet light mutagenesis

Abstract

Introduction into Escherichia coli WP2 bacteria of a mutation in the gyrB locus previously shown to reduce the degreee of chromosomal superhelicity caused a small decrease in the frequency of UV-induced mutations to streptomycin resistance (but not significantly) and to tryptophan independence (mostly ochre suppressors) in excision repair-proficient bacteria. It did not influence the ‘broth effect’ or the rate or extent of ‘mutation frequency decline’ of suppressor mutations. In an excision-deficient ( uvrA 155) background the yield of UV-induced streptomycin-resistant mutations was lower in gyrB bacteria at all doses; the yield of tryptophan-independent mutations was slightly lower at low doses and slightly higher at high doses. In both excision-proficient and -deficient the yield of UV-induced mutations to rifampicin resistance was apparently in gyrB mutants but this could be due at least in part to a hypersensitivity of some Rif r gyrB bacteria to UV. The number of spontaneous tryptophan-independent mutations was lower in gyrB bacteria but this was almost certainly due to their poorer viability on trypophan-limiting plates and not to a lower spontaneous mutation rate. In a temperature-sensitive presumed gyrase-deficient strain a small decrease in mutant yield at low doses was observed following incubation at restrictive temperature befor UV. This was ascribed to an enhancement of excision repair. Our failure to find any significant effect of gyrB mutation does not support the hypothesis that hairpin formation (which should be dependent on a high degreee of superhelicity) is involved in determining the ‘broth effect’, ‘mutatiion frequency decline’ or the probability that a mutation will occur spontaneously. Dramatic effects of superhelicity on UV mutagenesis also seem to be unlikely.