DETERMINATION OF THE EFFECT OF THE MUTATION IN TS20, A DNA MUTANT OF BALB/3T3 CELLS ON DNA TOPOISOMERASE Ⅱ

A temperature-sensitive mutant of Balb/3T3 cells, ts20, is defective in an early step of DNA chain elongation. Previous work has shown an association of the defect with the activity of DNA topoisomerase Ⅰ. In the studies presented herein we looked for evidence implicating the activity of DNA topoisomerase Ⅱ. Mutant, revertant and wild-type cells were equally sensitive to killing by novobiocin, a drug that inhibits the activity of DNA topoisomerase Ⅱ. The degree of inhibition of cell DNA synthesis in vivo and in vitro in mutant cells at restrictive temperature was also unaltered by the presence of the drug. The mutation had no effect on cell chromatin conformation or on supercoiling of mature polyomavirus DNA molecules. Our results provide evidence that DNA topoisomerase Ⅱ is not affected by the mutation in ts20.