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

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Abstract
The topoisomerase III gene (top3+) from Schizosaccharomyces pombe was isolated and a targeted gene disruption (top3::kanR) was used to make a diploid strain heterozygous for top3+. The diploid was sporulated and the top3::kanR 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-K547I, 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::kanR 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.

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