Mutagenic DNA repair in Escherichia coli. XV. Mutation frequency decline of ochre suppressor mutations in umuC and lexA bacteria occurring between ultraviolet irradiation and delayed photoreversal.

Tryptophan-independent mutations were induced in CM1141 trpE65 umuC122::Tn5 following exposure to ultraviolet light (UV) plus delayed photoreversal. The mutations appeared to be exclusively class 2 ochre suppressors and showed mutation frequency decline (MFD) when the bacteria were incubated in glucose-salts medium after UV and before photoreversal. The phenomenon was similar to MFD after normal UV mutagenesis of umu+ bacteria, being inhibited in the presence of caffeine or in the absence of glucose. Mutations were also induced by UV plus delayed photoreversal in the lexA (Ind-) strain CM561, and the yield was higher than in the umuC strain suggesting that potentially mutagenic configurations might be removed or altered to some extent by the product of a gene under lexA control such that fewer were available for misincorporation events. MFD was also demonstrated in CM561 showing that this process is not dependent on the derepression of any genes under lexA control. MFD could still be demonstrated 23 min after UV at a time when most misincorporations seem to have occurred, but for technical reasons the possibility could not be excluded that the misincorporations in question could have occurred during rather than before the exposure to photoreversing light. Delayed photoreversal mutagenesis of normally non-UV-mutable strains has been interpreted as a first stage (misincorporation) of normal UV mutagenesis. The present results are consistent with this interpretation since MFD of suppressor mutations is a feature of both processes.