The essential role of yeast topoisomerase III in meiosis depends on recombination

Yeast cells mutant for TOP3, the gene encoding the evolutionary conserved t ype I-5' topoisomerase, display a wide range of phenotypes including altere d cell cycle, hyper-recombination, abnormal gene expression, poor mating, c hromosome instability and absence of sporulation, In this report, an analys is of the role of TOP3 in the meiotic process indicates that top3 Delta mut ants enter meiosis and complete the initial steps of recombination. However , reductional division does not occur. Deletion of the SPO11 gene, which pr events recombination between homologous chromosomes in meiosis I division, allows top3 Delta mutants to form viable spores, indicating that Top3 is re quired to complete recombination successfully. A topoisomerase activity is involved in this process, since expression of bacterial TopA in yeast top3 Delta mutants permits sporulation. The meiotic block is also partially supp ressed by a deletion of SGS1, a gene encoding a helicase that interacts wit h Top3. We propose an essential role for Top3 in the processing of molecule s generated during meiotic recombination.