Figure 1.

Transcriptional silencing is uniformly high within HML but sharply decreases beyond the I silencer. (A) Schematic representation of the HML and HMR loci and their silencers on chromosome III. CEN, centromere. The open box in the HML E silencer represents the D-element (29). (B) The pattern of transcriptional silencing within and near HML. (Left) Strains used. The 11- to 18-kb fragment of chromosome III, including HML, is shown. The HML I silencer (I in open box) corresponds to position 14561–14701 (22). The filled bar to the right of HML I is 14702–14838. URA3 replaces 12015–12535 (XbaI–XbaI) in Y1861. Strains Y1995–YXB81 have URA3 inserted at the PvuII (P, 14441), SnaBI (S, 15016), EcoRV (E, 15410), BamHI (Ba, 16263), and BglII (Bg, 17344) sites, respectively, in both orientations. Copies of URA3 in these strains are designated 1–10, according to the positions of the URA3 promoter relative to HML I. X, XbaI site. (Right) Growth phenotypes. Cells of each strain were grown to late logarithmic phase, and serial dilutions (10-fold) were spotted on test plates and allowed to grow for 3 days. SC, synthetic complete medium; −Ura, SC medium lacking uracil; FOA, SC medium containing 5-fluoroorotic acid (5-FOA).