Unusual sensitivity to bleomycin and joint resistance to 9-beta-D-arabinofuranosyladenine and 1-beta-D-arabinofuranosylcytosine of mouse FM3A cell mutants with altered ribonucleotide reductase and thymidylate synthase.

This paper describes unusual sensitivity to bleomycin; resistance to aphidicolin, 9-beta-D-arabinofuranosyladenine, 1-beta-D-arabinofuranosylcytosine, and hydroxyurea; and a mutator phenotype as common phenotypes of two classes of previously reported mouse FM3A cell mutants with distinct biochemical defects. One class of mutants, selected for aphidicolin resistance, contains altered ribonucleotide reductase desensitized to deoxyadenosine triphosphate, a negative allosteric effector, and the other class contains temperature-sensitive thymidylate synthase. Abnormal levels of deoxyribonucleoside triphosphate pools, especially those of deoxyadenosine triphosphate, were the common biochemical trait for both classes of mutants. The aphidicolin-resistant thymidylate synthase mutants, isolated independently of aphidicolin resistance, showed reduced levels of thymidylate synthase and increased levels of ribonucleotide reductase. Bleomycin was more lethal to both classes of mutants than to wild-type cells, as determined by colony-forming ability. It also caused more marked scissions of DNA in the ribonucleotide reductase mutant cells than in the wildtype cells as analyzed by sedimentation on an alkaline sucrose gradient. Thus, the activity or level of ribonucleotide reductase seems to be one factor determining the bleomycin sensitivity of cells by potentiating the capacity of bleomycin to cleave DNA strands.