The sites of bleomycin-induced cleavage of purified and intracellular simian virus 40 DNA were examined. Breaks in purified DNA were mapped to several discrete sites that were distributed throughout the viral genome, but were not associated with a conmnon genetic element. Double-stranded breaks were made in positions of the first single-stranded nick, and regions of cuts were unaffected by changes In DNA superhelicity. Bleomycin cut intracellular chromosomes at the same sites that were cleaved in purified DNA. These results indicate that SV40 DNA contains DNA secondary structures that are highly preferred sites for BLM cleavage. These conformations appear to be unaffected by nucleoproteins bound to DNA.

Mutants of Chinese hamster ovary (CHO) cell resistant to cytosine arabinoside (ara-C), an inhibitor of DNA synthesis and antitumour drug, have been isolated and characterized both biochemically and genetically. Mutants occurring spontaneously and those induced by treatment with 2V r -methyl-2V y -Nitro-.A r -nitrosoguanidine (MNG), were obtained at a frequency of 0-24 x 10~6 and 3-4 x 10~6 respectively. The mutants showed a stable ara-C resistant phenotype which was inherited as a dominant trait in genetic crosses. The wild type (CHO K-1) and the mutant (103, 002 and 003) cells showed no differences in the levels of the uptake of ara-C or of its degradation. Results of biochemical studies further excluded the involvement of deaminase, kinase and ribonucleotide reductase as the possible factor(s) in conferring drug resistance to the mutant cells. However, the wild type and mutant DNA polymerases differed in the level of the in vitro incorporation of specific dNMP in the presence of ara-CTP. These data suggested that the wild-type DNA polymerase which becomes error prone in the presence of ara-CTP may cause the drug sensitivity of the wild-type cells and that a change in the mutant enzyme making it resistant (or less prone) to ara-CTP induced errors in dNMP incorporation may control the drug resistance of the mutant cells.