Interaction between yeast sgs1 helicase and DNA topoisomerase III

The Saccharomyces cerevisiae Sgs1 protein is a member of the RecQ family of DNA helicases that includes the human Bloom's syndrome and Werner's syndrome proteins. In this work, we report studies on the interaction between Sgs1 and DNA topoisomerase III in vitro and in vivo. Affinity chromatography experiments with various fragments of Sgs1, a 1447-amino acid polypeptide, suggested that its N-terminal one-fifth was sufficient for interaction with DNA topoisomerase III. Gel electrophoretic mobility shift assays also indicated that a fragment Sgs1(1-283), containing residues 1-283, inhibited the binding of DNA topoisomerase III to single-stranded DNA. A shorter protein fragment containing residues 1-107 also showed partial inhibition in these assays. Studies of a sgs1 top1 double mutant lacking both Sgs1 and DNA topoisomerase I showed that the slow growth phenotype of this double mutant is suppressed by expressing full-length Sgs1, but not Sgs1 without the N-terminal 107 amino acid residues. In sgs1 top3 cells devoid of DNA topoisomerase III, however, expression of full-length Sgs1 or Sgs1 lacking the N-terminal 107 amino acid residues has the same effect of reducing the growth rate of the double mutant. These in vitro and in vivo data indicate that Sgs1 and DNA topoisomerase III physically interact and that this interaction is physiologically significant.