Postreplication repair (PRR) was quantified in normal human fibroblasts and in
xeroderma pigmentosum (XP) variant fibroblasts
after treatment with UV or
benzo[a]pyrene diol
epoxide-I (
BPDE-I). PRR may be defined as the elimination of discontinuities in the daughter-strand
DNA and the replicative bypass of lesions in the
DNA template. Pathways of PRR reduce the number of
DNA growing points that are blocked at template lesions and increase the rate of growth of nascent
DNA on damaged templates. Rates of
DNA synthesis and strand growth were measured in
solvent- and
carcinogen-treated cells by velocity sedimentation analyses of radiolabeled nascent
DNA in alkaline
sucrose gradients. Logarithmically growing normal fibroblasts displayed D0 values of 6.3 J/m2 and 0.37 microM for the inhibition of
DNA synthesis in active replicons by UV and
BPDE-I, respectively. Under identical conditions, the XP variant cells exhibited D0 values of 1.5-2.0 J/m2 and 0.27-0.31 microM. Pulse-chase experiments were performed in cells synchronized at the beginning of the S phase. Normal and XP variant cells displayed inhibition of
DNA strand growth by UV, with D0 values of 21.6 and 7.0 J/m2, respectively. The D0 values for the inhibition of
DNA strand growth by
BPDE-I were 0.85 microM for the normal cells and 0.62-0.79 microM for the XP variant cells. The inhibitions of DNA replication by UV and
BPDE-I were also analyzed in terms of DNA lesion frequencies. Based on the D0 values for inhibition of DNA replication, we concluded that the XP variant cells express maximally 25-33% of the total PRR activity observed in normal fibroblasts after UV treatment. Conceivably, this deficiency in PRR activity results in the XP variant's increased risk of
cancers induced by sunlight, because XP variant cells and normal fibroblasts are equally proficient in excision repair. Both normal and XP variant fibroblasts, however, displayed similar PRR activities in response to
BPDE-I treatment.