DNA topoisomerase II is required for the G 0 ‐to‐S phase transition in Drosophila Schneider cells, but not in yeast

We previously reported that DNA topoisomerase II (topo II) is required for the G 0 ‐to‐S phase transition in mammalian cells [Hossain et al. (2002) ICRF‐193, a catalytic inhibitor of DNA topoisomerase II, inhibits re‐entry into the cell division cycle from quiescent state in mammalian cells. Genes Cells 7, 285–294]. In this study, we examined whether the requirement for topo II is evolutionarily conserved in Drosophila and yeast. ICRF‐193, a catalytic inhibitor of topo II, inhibited DNA synthesis in Drosophila Schneider cells released from the G 0 (stationary) phase, whereas the drug did not inhibit DNA synthesis in Schneider cells released from the M phase. Depletion of topo II mRNA by RNA‐interference (RNAi) in G 0 ‐phase Schneider cells resulted in significant inhibition of DNA synthesis after release from G 0 ‐arrest. In the yeast topo II temperature‐sensitive (ts) mutant, the initial cycle of DNA synthesis occurred at a restrictive temperature after release from starvation‐induced G 0 phase and doubling of the DNA content in the cells was confirmed by both flow cytometry and fluorescence spectrophotometry. DNA synthesis in yeast cells after release from the G 0 phase was also observed in the presence of ICRF‐193. Doubling of the DNA content was observed during spore germination of topo II ts mutant yeast at a restrictive temperature as determined by fluorescence spectrophotometry. These results indicate that topo II is required for the G 0 ‐to‐S phase transition in Drosophila Schneider cells, but not in yeast.