Mutagenesis after exposure of bacteria to ultraviolet light and delayed photoreversal

Summary The induction of mutations by ultraviolet light and delayed photoreversal in bacteria defective for SOS mutagenesis is discussed in terms of two models: the two-step misincorporation and bypass model, and the model involving simple deamination of cytosine-containing dimers. In phage S13 the latter appears to be the predominant mechanism. In Escherichia coli there is little evidence that the simple deamination mechanism is of any significance except in ung strains lacking uracil glycosylase where uracils left after photoreversal are not removed. Deamination might, however, occur during the operation of translesion synthesis via the two-step model and if it did, subsequent photoreversal would lead to the mutation being extended from one to both strands by uracil glycosylase repair rather than being removed.