DNA mutagenesis and repair in UV-irradiated E. coli K-12 under condition of mutation frequency decline.

This paper shows that mutation frequency decline (MFD) occurs in UV-irradiated and transiently starved Escherichia coli K-12 strain AB1157. This effect involves preferential repair of pre-mutagenic lesions situated on the transcribed strand, and is mfd-, and uvrA-dependent. Mutations were tested by measuring reversion of argE3(ochre) to Arg+ and the types of reversions and the mutational specificity were defined. UV-irradiation induces reversions to Arg+ predominantly by forming ochre suppressors, supB and, much more rarely, supE(ochre). In irradiated and then transiently starved cells, the ratio of supB/supE(ochre) is reversed, and supE(ochre) suppressors predominate. Back reversions at the argE3 site occurs only in bacteria defective in UvrABC-excinuclease. The introduction of a Tn10 transposon makes bacteria more sensitive to UV-irradiation and diminishes the frequency of reversions. Transformation with pGW2123, a umuD'C-bearing plasmid, recovers and enhances the frequency of reversion but has no effect on post-UV-survival of the bacteria.