A model depicting potential interactions between Gcn5 and Hda1. (A) The HAT Gcn5 and the HDAC Hda1 have opposing functions that individually benefit APC function. (B) and (C) Gcn5 and Hda1 appear to compete for Tup1 binding. (B) If Hda1 first gains access to the promoter, recruitment of Gcn5 is partially blocked. (C) Under conditions where gene transcription must be derepressed, Tup1 may recruit Gcn5 to the promoter to prime transcriptional initiation, thus displacing Hda1. The protein labelled × represents a DNA binding factor that recruits the Tup1/Ssn6 corepressor complex to silent genes.