Topoisomerase I-dependent viability loss in Saccharomyces cerevisiae mutants defective in both SUMO conjugation and DNA repair

Siz1 and Siz2/Nfi1 are the two Siz/PIAS SUMO E3 ligases in Saccharomyces cerevisiae. Here we show that siz1 Delta siz2 Delta mutants fail to grow in the absence of the homologous recombination pathway or the Fen1 ortholog RAD27. Remarkably, the growth defects of mutants such as siz1 Delta siz2 Delta rad52 Delta are suppressed by mutations in TOP1, suggesting that these growth defects are caused by topoisomerase I activity. Other mutants that affect SUMO conjugation, including a ulp1 mutant and the nuclear pore mutants nup60 Delta and nup133 Delta, show similar top1-suppressible synthetic defects with DNA repair mutants, suggesting that these phenotypes also result from reduced SUMO conjugation. siz1 Delta siz2 Delta mutants also display TOP1-independent genome instability phenotypes, including increased mitotic recombination and elongated telomeres. We also show that SUMO conjugation, TOP1, and RAD27 have overlapping roles in telomere maintenance. Top1 is sumoylated, but Top1 does not appear to be the SUMO substrate involved in the synthetic growth defects. However, sumoylation of certain substrates, including Top1 itself and Tril (YMR233W), is enhanced in the absence of Top1 activity. Sumoylation is also required for growth of top1 Delta cells. These results suggest that the SUMO pathway has a complex effect on genome stability that involves several mechanistically distinct processes.