FIG. 4.

Kinetics of mating type switching. A schematic representation of the MATa and MATα loci indicating the locations of BamHI and StyI sites and the hybridization probe is shown at the top of the figure. HO endonuclease produces a 0.7-kb fragment from the 0.9-kb MATa StyI fragment. A 1.8-kb StyI fragment is produced when the mating type switches from MATa to MATα. Extensive 5′-to-3′ degradation from the HO-cut site through the distal BamHI and StyI sites leads to the appearance of high-molecular-weight BamHI/StyI fragments. Double digestions with StyI and BamHI were performed because the StyI site at position 5450 (67) is absent in W303 strains. DNA was isolated from cultures prior to galactose induction (0-h time point) and at 1-h intervals after HO induction. The arrows to the left of the gel indicate the high-molecular-weight fragments generated by nuclease digestion from the HO-cut site. ssDNA, single-stranded DNA.