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 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.
Bates
H
H
Medical Research Council Cell Mutation Unit, University of Sussex, Brighton, England.
Randall
S K
SK
Rayssiguier
C
C
Bridges
B A
BA
Goodman
M F
MF
Radman
M
M
D004256:DNA Polymerase I / Q000235:genetics*; D004260:DNA Repair; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics*; D009154:Mutation / Q000528:radiation effects*; D012293:Rifampin / Q000494:pharmacology; D013014:SOS Response, Genetics; D014466:Ultraviolet Rays; D014633:Valine / Q000378:metabolism
D004256:DNA Polymerase I; D014633:Valine; D012293:Rifampin