Figure 7.

Biological role of Cyc8–Tup1 and a comparison with Gal80. (A) Regulation of Gal4 activation by Gal80. Under repressing conditions (e.g., growth in the absence of galactose), the activation domain of Gal4 is blocked by a physical interaction with Gal80. Upon growth in glucose-limiting medium containing galactose, Gal80 dissociates from Gal4, exposing its activation domain for recruiting coactivators. (B) Cyc8–Tup1 is recruited to target promoters by a class of “repressor–activator” proteins. Under normal (repressing) conditions, Cyc8–Tup1 blocks the activating potential of these proteins by interacting with and masking their activation domains (ADs). Upon derepressing conditions, the responsive repressor–activator is modified, leading to a conformational change in the interaction with Cyc8–Tup1 such that the activation domain can now recruit the Swi/Snf, SAGA, and Mediator coactivators. Cyc8–Tup1 remains at the promoter and contributes to the activation process. During reinitiation of repression, Cyc8–Tup1 re-establishes a repressive state through removing bound coactivators at promoters. The acetylation on histone H3 is removed by global nontargeted activity of deacetylases, and nucleosomes are subsequently redeposited to promoters.