Figure 2.

sgs1Δ exo1Δ mutants have mating-type switching defects. (A) Time course of DSB repair at the MAT locus in the indicated strains. Galactose was added at time 0 to induce HO expression, and samples were removed at 1-h intervals for Southern blot analysis. Bands corresponding to uncut (MATa), HO endonuclease-cut and product (MATα) restriction fragments are indicated. The unlabeled band corresponds to the distal fragment and serves as a loading control. (B) The catalytic activities of Sgs1 and Exo1 promote mating-type switching. sgs1Δ exo1Δ cells transformed with an empty vector, or with a vector expressing the wild-type (pSGS1) or a helicase-deficient (psgs1-hd) version of Sgs1 (top panel), or with a vector expressing the wild-type (pEXO1) or a nuclease-deficient (pexo1-nd) version of Exo1 (bottom panel) were analyzed as in A. (C) Mre11 and Sgs1 promote mating-type switching by independent pathways. Analyses were as in A.