Figure 2.—

Moving RE to another location on the left arm of chromosome III does not impair its function. (A) Physical map of chromosome III showing the loci involved in this experiment and their position in kilobases. This map is not to scale and the dotted lines represent interruptions in the sequence of the chromosome. (B) Usage of HMLα or of HMRα-LEU2 inserted at different positions on the left arm of chromosome III (41 kb, HIS4, or LEU2) in strains deleted for HMLα (Wu et al. 1996). The donors are represented by shaded horizontal bars. The vertical shaded bars represent the left donor usage as the percentage of a population of switched cells, determined on Southern blots. The solid square represents RE sequences. The solid circle represents the centromere. The horizontal solid rectangle represents MATa. (C) Same as B but the cells are deleted for RE (Δ) (Wu et al. 1996). (D) Same as B but an additional RE is inserted at position 74 kb. (E) Same as D but RE at 29 kb has been deleted. HML usage was measured at least three times on independent Southern blots except when HMRα-LEU2 was inserted at LEU2. Means and standard errors are shown. Strains used in these experiments: (B and C) Wu et al. (1996); (D) 12 kb: GFR18, 41 kb: GFR14, 66 kb: GFR15; (E) 12 kb: GFR22, 41 kb: GFR17, 66 kb: GFR24. (F) Representative Southern blot analysis of the product of mating-type switching in strains bearing either HMLα or (in strains deleted for HMLα) the HMRα donor inserted at different positions on the left arm of chromosome III (Wu et al. 1996). HML usage was quantified on Southern blots after digestion of the genomic DNA with the BamHI and StyI restriction enzymes. A MAT-distal probe detects the products MATα and MATα-BamHI, as well as the parental MATa sequence and a second MAT distal fragment. The extra band observed in the GFR17 corresponds to the unrepaired HO-cut fragment in a fraction of the population.