REPAIR OF DAUGHTER STRAND GAPS IN NASCENT DNA FROM MOUSE EPIDERMAL-CELLS TREATED WITH DIHYDRODIOL EPOXIDE DERIVATIVES OF BENZO[A]PYRENE

Alkaline sucrose gradient analysis of [methyl-3H]thymidine-pulse-labeled DNA was used to study the effect of (±)-7β,8a-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (benzo[a]pyrene-diol epoxide I), a potent mutagen and carcinogen, and (±)-7β,8α-dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (benzo[a]pyrene-diol epoxide II), a weaker mutagen and carcinogen, on the size of newly synthesized DNA in primary cultures of mouse epidermal cells. Both isomers caused a dose-dependent decrease in the size of newly synthesized DNA and in the rate of [methyl-3H]thymidine incorporation into DNA. When the pulse time was increased in the treated cells so that the amount of [methyl-3H]thymidine incorporation was equal to the control, newly synthesized DNA from exposed cells was still considerably smaller than DNA from control cells. The low molecular weight of the nascent DNA from treated cells was consistent with, but not indicative of, the presence of gaps in the nascent DNA from the treated cells. Evidence of gapped DNA synthesis was obtained by treatment of extracted DNA with a single-strand specific endonuclease from Neurospora crassa. The endonuclease treatment did not significantly alter the profile of [methyl-3H]thymidine prelabeled DNA from benzo[a]pyrene-diol epoxide-treated cultures but did introduce double-strand breaks in pulse-labeled DNA from treated cultures. The numbers of [14C]benzo[a]pyrene-diol epoxide I or [3H]benzo[a]pyrene-diol epoxide II-DNA-bound adducts and daughter strand gaps were compared at several dose levels. Treatment with either isomer yielded one gap in the nascent DNA DNA-bound adduct. Pulse-chase experiments showed that gaps in the nascent DNA were closed with time. © 1979.