Fig. 3. A model of mediator function. This schematic model suggests that particular combinations of activators play a role in influencing the conformation of mediator. These different conformations would influence the re-entry of RNA polymerase II to the promoter to initiate subsequent rounds of transcription. The combination of activators in panel A forces a mediator conformation that only promotes the slow re-entry of RNA polymerase II to the promoter, while the combination in panel B promotes a faster RNA polymerase II re-entry via a more productive interaction with mediator. It is important to note that activator A, in the context of a different set of activators (X and Y in panel B), is capable of inducing a different conformation of mediator (Lefstin and Yamamoto, 1998). This is not meant to suggest that the number of activators is the key point in inducing a particular mediator conformation.