We showed previously that in repair-proficient human cells the location of the premutagenic lesion induced by (+-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo(a)
pyrene (
BPDE), namely, the
guanine in a G.C base substitution, in mutants derived from cells treated at the beginning of S phase just when the
hypoxanthine (guanine) phosphoribosyltransferase gene is replicated, differs significantly from their location in cells treated 12 h prior to the beginning of S phase (early G1 phase) (R-H. Chen et al., Proc. Natl. Acad. Sci. USA, 87:8680-8684, 1990). This suggests that the cells preferentially remove
BPDE adducts from the transcribed strand. We have now determined the kinds and location of independent mutations induced by
BPDE in the coding region of the
hypoxanthine (guanine) phosphoribosyltransferase gene of synchronized repair-deficient
xeroderma pigmentosum cells (XP12BE, complementation group A), treated at S or in G1. Nineteen of 25 mutants derived from S-treated cells and 23 of 28 mutants from G1-treated cells contained base substitutions. Eighty-nine percent of these involved a G.C base pair, primarily G.C----T.A transversions. This is similar to the kinds of mutations we saw in the repair-proficient cells. However, in contrast to our earlier results, there was no change in strand distribution of premutagenic
BPDE lesions. In both populations, approximately 26% of the base substitutions involving G.C base pairs had the G located in the transcribed strand, 5 of 18 in the S phase mutants, and 5 of 21 in the G1 phase mutants. These results support the hypothesis that the strong strand bias of induced mutations observed in the repair-proficient cells results from preferential repair of
BPDE-induced DNA damage from the transcribed strand.