Figure 4.—

Physical analysis of DSB formation at FS2 in strains deficient for various nucleases. All strains were grown overnight in high galactose medium and then washed in water and resuspended in medium lacking galactose for 6 hr. DNA was extracted and chromosomal DNA molecules were separated by gel electrophoresis as described in materials and methods. The separated molecules were examined by Southern analysis, using a probe derived from the left end of III. The ratio of chromosome III molecules broken at FS2 (180-kb fragment) vs. the intact III (330 kb) was quantitated using a PhosphoImager. MS71 is a wild-type haploid strain, and GAL-POL1 ty1Δ is isogenic with the GAL-POL1 strain except that it lacks one of the two Ty1 elements that compose FS2 (Lemoine et al. 2005).