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

Abstract

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::kan(R) 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- top3- mutants compared with rad12 single mutants suggests that Top3 also functions independently of Rad12.