Mechanisms of activation and repression of eukaryotic gene expression. (a) Transcription activators and coactivators bound to distant regulatory sites (enhancers and UASs) recruit components of the Pol II general (basal) transcription machinery to the promoter. Coactivators such as Mediator and TFIID are required at essentially all promoters. They function as a bridge between activators and the polymerase, and do not interact with DNA directly. (b) Repression is mediated by proteins that disrupt or prevent essential contacts between Pol II and activators or coactivators. See Figure 21-