RESEARCH ARTICLE Genome-Destabilizing Effects Associated with Top1 Loss or Accumulation of Top1 Cleavage Complexes in Yeast

Topoisomerase 1 (Top1), a Type IB topoisomerase, functions to relieve transcription- and replication-associated torsional stress in DNA. We investigated the effects of Top1 on ge-nome stability in Saccharomyces cerevisiae using two different assays. First, a sectoring assay that detects loss of heterozygosity (LOH) on a specific chromosome was used to measure reciprocal crossover (RCO) rates. Features of individual RCO events were then molecularly characterized using chromosome-specific microarrays. In the second assay, cells were sub-cultured for 250 generations and LOH was examined genome-wide using microarrays. Though loss of Top1 did not destabilize single-copy genomic regions, RCO events were more complex than in a wild-type strain. In contrast to the stability of single-copy regions, sub-culturing experiments revealed that top1mutants had greatly elevated levels of instability within the tandemly-repeated ribosomal RNA genes (in agreement with previous results). An intermediate in the enzymatic reaction catalyzed by Top1 is the cova-lent attachment of Top1 to the cleaved DNA. The resulting Top1 cleavage complex (Top1cc) is usually transient but can be stabilized by the drug camptothecin (CPT) or by th