A mutant allele of
SGS1 of
Saccharomyces cerevisiae was identified as a suppressor of the slow-growth phenotype of
top3 mutants. We previously reported the involvement of Top3
via the interaction with the N-terminal region of Sgs1 in the complementation of methylmethanesulfonate (MMS) sensitivity and the suppression of hyper recombination of a
sgs1 mutant. In this study, we found that several amino acids residues in the N-terminal region of Sgs1 between residues 4 and 33 were responsible for binding to Top3 and essential for complementing the sensitivity to MMS of
sgs1 cells. Two-hybrid assays suggested that the region of Top3 responsible for the binding to Sgs1 was bipartite, with portion in the N- and C-terminal domains. Although disruption of the
SGS1 gene suppressed the semi-lethality of the
top3 mutant of strain MR, the
sgs1-top3 double mutant grew more slowly and was more sensitive to MMS than the
sgs1 single mutant, indicating that Top3 plays some role independently of Sgs1. The DNA topoisomerase activity of Top3 was required for the Top3 function to repair DNA damages induced by MMS, as shown by the fact that the
TOP3 gene carrying a mutation (Phe for Tyr) at the amino acid residue essential for its activity (residue 356) failed to restore the MMS sensitivity of
sgs1-top3 to the level of that of the
sgs1 single mutant. Epistatic analysis using the
sgs1-top3 double mutant,
rad52 mutant and
sgs1-top3-rad52 triple mutant indicated that
TOP3 belongs to the
RAD52 recombinational repair pathway.
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