BENZO[A]PYRENE-DNA ADDUCTS INHIBIT THE DNA HELICASE ACTIVITY OF THE BACTERIOPHAGE-T7 GENE-4-PROTEIN

The gene 4 protein of bacteriophage T7 provides the essential helicase and primase activities for the replication of the T7 genome. In addit ion, it also displays a DNA-dependent deoxyribonucleoside triphosphata se activity, the preferred substrate of which is dTTP. Previous invest igations have demonstrated that the translocation of the gene 4 protei n along single-stranded DNA is blocked by the presence of benzo[alpha] pyrene-DNA adducts and that the gene 4 protein is likely to be sequest ered at the sites of these adducts. In the present study, we directly show that the helicase activity of the gene 4 protein is also profound ly inhibited by the benzo[alpha]pyrene-DNA adducts. The inhibitory eff ects of these adducts are strand-specific in that they block the DNA h elicase activity of the gene 4 protein only when they are located in t he DNA strand where the gene 4 protein translocates when it unwinds do uble-stranded DNA. Consistent with the hypothesis that the gene 4 prot ein is sequestered at the adduct site, we also show that the complexes formed by the gene 4 protein and benzo[alpha]pyrene-modified DNA are far more stable than those formed by the gene 4 protein and unmodified DNA.