Fig. 4. Enhanceosome versus Information Display enhancer models. (A) In the enhanceosome model, the enhancer serves as an information processing center, receiving inputs from multiple transcription factors that bind it. A highly structured complex or enhanceosome, creates a stereospecific interface for docking with and recruiting the basal transcription machinery. Here the enhancer serves as a molecular computer, resolves multiple inputs and provides a single output to the basal transcription machinery. With such an enhancer, the target gene would be activated only upon the assembly of a complex, providing a precise on/off binary transcriptional switch in response to the appropriate stimulus. Graded responses from such an element could be achieved by varying the stability of the entire complex, possibly in response to activator concentrations. (B,C) Information Display or "Billboard' enhancer. Rather than acting as a central processing unit, subelements can display contrasting information, which is then interpreted by basal transcription machinery. In this model, the basal machinery `samples' discrete regions of the enhancer each composed of a small number of transcription factor binding sites, either iteratively (B) or simultaneously (C). Successive/multiple interactions with the basal machinery, and the biochemical consequence of these interactions, would dictate the overall output of the enhancer.