DEPENDENCE OF MAMMALIAN DNA-SYNTHESIS ON DNA SUPERCOILING .3. CHARACTERIZATION OF THE INHIBITION OF REPLICATIVE AND REPAIR-TYPE DNA-SYNTHESIS BY NOVOBIOCIN AND NALIDIXIC-ACID

Novobiocin and nalidixic acid, inhibitors of the bacterial enzyme DNA gyrase, inhibit DNA, RNA and protein synthesis in several human and rodent cell lines. The sensitivity of DNA synthesis (both replicative and repair) to inhibition by novobiocin and nalidixic acid is greater than that of protein synthesis. Novobiocin inhibits RNA synthesis about half as effectively as it does DNA synthesis; nalidixic acid inhibits both equally well. Replicative DNA synthesis, as measured by incorporation of [3H]thymidine, is blocked by novobiocin in a number of cell strains [diploid human fibroblast cell strains CRL 1295 and CRL 1187, the xeroderma pigmentosum strain CRL 1223, and Chinese hamster ovary cell strain]; the inhibition is reversible with respect to both DNA synthesis and cell killing, and continues for as long as 20-30 h if the cells are kept in novobiocin-containing growth medium. Both novobiocin and nalidixic acid inhibit repair DNA synthesis (measured by BND-cellulose chromatography) induced by UV light or N-methyl-N''-nitro-N-nitrosoguanidine (but not that induced by methyl metanesulfonate) at lower concentrations (as low as 5 .mu.g/ml) than those required to inhibit replicative DNA synthesis (50 .mu.g/ml or greater). Neither novobiocin nor nalidixic acid alone induces DNA repair synthesis. Incubation of UV-irradiated cells with 10-100 .mu.g/ml novobiocin results in little, if any, further reduction of colony-forming ability (beyond that caused by the UV irradiation). Novobiocin at sufficiently low concentrations (200 .mu.g/ml) apparently generates a quiescent state (in terms of cellular DNA metabolism) form which recovery is possible. Under more drastic conditions of time in contact with cells and concentration, novobiocin itself induces mammalian cell killing.