Spontaneous and UV-induced mutations in Escherichia coli K-12 strains with altered or absent DNA polymerase I.


The induction of mutations to valine resistance and to rifampin ...
The induction of mutations to valine resistance and to rifampin resistance occurs after UV irradiation in bacteria carrying a deletion through the polA gene (delta polA), showing that DNA polymerase I (PolI) is not an essential enzyme for this process. The PolI deletion strain showed a 7- to 10-fold-higher spontaneous mutation frequency than the wild type. The presence in the deletion strain of the 5'----3' exonuclease fragment on an F' episome caused an additional 10-fold increase in spontaneous mutation frequency, resulting in mutation frequencies on the order of 50- to 100-fold greater than wild type. The mutator effect associated with the 5'----3' exonuclease gene fragment together with much of the effect attributable to the polA deletion was blocked in bacteria carrying a umuC mutation. The mutator activity therefore appears to reflect constitutive SOS induction. Excision-proficient polA deletion strains exhibited increased sensitivity to the lethal effect of UV light which was only partially ameliorated by the presence of polA+ on an F' episome. The UV-induced mutation rate to rifampin resistance was marginally lower in delta polA bacteria than in bacteria carrying the polA+ allele. This effect is unlikely to be caused by the existence of a PolI-dependent mutagenic pathway and is probably an indirect effect caused by an alteration in the pattern of excision repair, since it did not occur in excision-deficient (uvrA) bacteria. An excision-deficient polA deletion strain possessed UV sensitivity similar to that of an isogenic strain carrying polA+ on an F' episome, showing that none of the functions of PolI are needed for postreplication repair in the absence of excision repair. Our data provide no evidence for a pathway of UV mutagenesis dependent on PolI, although it remains an open question whether PolI is able to participate when it is present.




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