FIG. 4.

Overexpression of the Ty5 TD disrupts telomeric silencing and loss of silencing is complemented by overexpression of Sir3p. Two reporter genes, URA3 and ADE2, are located at telomeres VIIL and VR, respectively (16). URA3 expression was measured by growth of the yeast cells on selective media. ADE2 expression is indicated by colony color; when ADE2 is repressed, the colonies are red or pink. (A) Overexpression of GBD-TD disrupts telomeric silencing (i.e., causes URA3 expression), in contrast to what occurs in its mutant form (GBD-td) and with GBD alone. (B) Overexpression of Sir2p and Sir3p strengthen silencing, and Sir4p breaks silencing, as previously reported (23, 39). This is demonstrated in the GBD control strains and GBD-td strains. When the TD and SIR genes are overexpressed in the same strains, Sir3p, but not Sir2p or Sir4p, restores silencing.