Figure 3.—

The centromere constitutes a barrier to RE activity. (A) Physical map of chromosome III showing the loci involved in this experiment and their positions in kilobases on the chromosome. (B) Effect of the centromere on RE function. The percentage of HMRα usage (vertical shaded bar) introduced at the LEU2 locus in strains deleted for HML has been measured with RE relocated to three different positions around the centromere: 112.5 kb (ECY139), 115 kb (ECY137), 116.5 kb (ECY138). ECY135 and ECY136 have HML at its normal location and with RE present or deleted, respectively. The solid circle represents the centromere. The solid squares represent RE. (C) To assess the negative effect of the centromere on RE function, the centromere has been moved to the right of RE when at position 116.5 kb (ECY169). Donor usage was quantified at least three times on independent Southern blots; means and standard errors are shown. (D) Representative Southern blot analysis of the product of mating-type switching in strains bearing the RE in the region of the centromere. Genomic DNA was digested with BamHI and HindIII restriction enzymes. Using a Yα fragment as a probe leads to the detection of the products MATα and MATα-BamHI.