Figure 4.

set1 mutants are defective in silencing. (A) SET1 was deleted in a strain containing a TRP1 reporter gene at HML. Compared with wild-type strains, the set1 mutant grows well in the absence of tryptophan and shows slightly decreased mating, demonstrating derepression of the HM silent mating-type loci. The strains shown are RS928 (MATα SET1), RS927 (MATa SET1), and LPY2546 (MATα set1Δ). The MATa mating tester lawn used was a227. (B) SET1 was deleted in a strain containing a URA3 telomeric reporter gene. Using the 5-FOA assay described in MATERIALS AND METHODS, URA3 expression was evaluated for two wild-type (UCC1001) and two set1 mutant (LPY1297) cultures. Growth is comparable on complete medium, but set1 strains are unable to grow on medium containing 5-FOA, demonstrating that the normally silenced telomeric URA3 reporter gene is transcribed.