Figure 1: Slow establishment of heterochromatic silencing by the H3K9 methyltransferase Clr4.

(a) Mating-type region. Nucleosomes between the IR-L and IR-R boundaries are methylated by Clr4, silencing gene expression. The chromosomal location of the (XbaI)::ura4⁺ reporter is shown. cenH: ∼4.2 kb region with centromere homology; grey boxes: ORFs; yellow boxes: silent mating-type information at mat2-P and mat3-M. (b) de novo silencing of (XbaI)::ura4⁺ by Clr4. Colonies formed by spores on rich medium were replicated onto medium lacking uracil. All colonies shown contain the (XbaI)::ura4⁺ reporter. A phenotypic gap is observed following re-introduction of clr4⁺into clr4Δ cells (middle panels) allowing clr4⁺cells to grow in the absence of uracil. The three colonies shown on the last row (clr4⁺) are genetically identical to the colonies above them (clr4Δ to clr4⁺) but their heterochromatin is fully established. (c) Dicer facilitates the establishment of heterochromatic silencing by Clr4, Clr7 and Clr8. Wild-type alleles were re-introduced into deletion strains by genetic crosses as indicated. Silencing of (XbaI)::ura4⁺ permits growth on 5-fluoroorotic acid (FOA). Establishement of (XbaI)::ura4⁺ silencing is more efficient in the dcr1⁺ than in the dcr1Δ progeny of each cross.