Figure 4.—

Effect of RE on donors located on chromosome V. (A) RE activates poorly a trans-located donor on chromosome V. Donor preference has been measured in strains derived from AN406 in which HMLα has been deleted from chromosome III and integrated near the left telomere of chromosome V at position 9 kb (AN409). The effect of RE on the donor located on chromosome V has been quantified in a strain in which RE has been introduced on chromosome V at position 24 kb (GF26). A representative Southern blot analysis, similar to that carried out in Figure 2, is shown. (B) RE is strongly active on the same chromosome V-located donor when the mating-type switching apparatus is moved entirely on chromosome V. In this experiment, HMLα, MAT, and HMRa have been deleted from chromosome III. HMRα has been reintroduced at position 9 kb on chromosome V, MATa at the ARG5,6 locus, and HMRα-BamHI at the KHS1 locus. HMRα usage has been measured in strains with (ECY429) or without (ECY427) RE at position 24 kb. A representative Southern blot analysis of the product of MAT switching is shown. Genomic DNA was analyzed as in Figure 3. In both experiments, donor usage was measured at least three times on independent Southern blots; means and standard errors are shown.