The top3+ gene is essential in Schizosaccharomyces pombe and the lethality associated with its loss is caused by Rad12 helicase activity

The topoisomerase III gene (top3(+)) from Schizosaccharomyces pombe was isolated and a targeted gene disruption (top3::kan(R)) was used to make a diploid strain heterozygous for top3(+), The diploid was-sporulated and the top3::kan(R) spores went through four to eight cell divisions before arresting as elongated, predominantly binucleated cells with incompletely segregated chromosomes. This demonstrates that top3(+) is essential for vegetative growth in fission yeast, The aberrant chromosomal segregation seen in top3::kanR cells is unlike the 'cut' phenotype seen in mitosis-defective mutants and so we refer to this phenotype as 'torn', A deletion mutant, rad12-hd (rad12 is a homolog of Saccharomyces cerevisiae SGS1), partially suppressed the lethality of top3 mutants. A point mutant, rad12-K5471, which presumably eliminates helicase activity, also suppresses the lethality of top3 mutants, demonstrating that the lethality seen in top3 cells is most likely caused by the helicase activity of Rad12. This double mutant grows very slowly and has much lower viability compared to rad12-hd top3::kan(R) cells, implying that the helicase activity of Rad12 is not the only cause of top3(-) lethality. The low viability of rad12 top mutants compared with rad12 single mutants suggests that Top3 also functions independently of Rad12.