A UmuD,C-dependent pathway for spontaneous G:C to C:G transversions in stationary phase Escherichia coli mut Y

In Escherichia coli trpA23 bacteria lacking the MutY glycosylase and incubated on plates in the absence of tryptophan, tryptophan-independent mutants continue to arise during incubation over many days. Their appearance is enhanced in umuD+,C+ strains in comparison with strains carrying a deletion through the umu operon and the umuD,C-dependent mutants were greater in number in uvrA bacteria (lacking nucleotide excision repair) than in uvr+ bacteria. Sequencing of mutations occurring in uvrA bacteria revealed the presence of G:C to C:G transversions but only in umuD+,C+ strains. There is thus a pathway in starved bacteria that generates G:C to C:G transversions and requires the inducible UmuD,C proteins. The data are consistent with the occurrence of a lesion, probably 8-oxoguanine, against which guanine may be incorporated during DNA synthesis by "dNTP stabilised" misalignment against the downstream template base. Upon realignment the configuration is substrate for MutY glycosylase which can remove the unmodified guanine. It is hypothesised that UmuD,C proteins are required for primer extension from the mismatch once formed.