Ultraviolet light induces different spectra of lacI sequence changes in vegetative and conjugating cells of Escherichia coli

We have analyzed the nucleotide sequence changes responsible for mutations from lacI s to lacI − induced in ultraviolet light-irradiated, excision-deficient cells. Irradiated cells were either used as donors in the conjugational transfer of an F′ lacI s plasmid to SOS-induced, excision-deficient recipients or allowed to continue vegetative growth. Although the types and proportions of premutagenic lesions are likely to have been very similar in these two circumstances, analysis of the sequence data shows that different spectra of mutations were induced. In vegetative cells there were about equal numbers of transitions and transversions, but transitions outnumbered transversions by about three to one in exconjugants. About 90% of the single nucleotide substitutions could be assigned to a bipyrimidine target sequence in both sets of data, but they differed with respect to the location of the substitution: more or less equal numbers were found at the 3′ and 5′ sites of the probable bipyrimidine target in vegetative cells, but in exconjugants over 80% were at the 3′ site. It is also possible that mutations were targeted more commonly at T-C sequences in exconjugants than in vegetative cells, but the evidence for this is less secure. We conclude that these results reflect some dissimilarity between vegetative cells and exconjugants in the way damaged DNA is replicated or lesions tolerated, but the particular features of these processes responsible for the different mutational spectra have not yet been identified.