The yeast mediator.
The large number of signaling pathways and regulatory proteins that affect transcription highlights a need for funneling of information since transcription of all protein encoding genes is executed by the same set of general transcription factors and RNA polymerase II. This demand is met by large protein complexes such as Mediator that interact with the basic RNA polymerase II machinery and thus adds diversity simply by increasing the surface that is exposed to the incoming signals. The recent description of Mediator-like complexes in metazoans identifies it as a key player in transcriptional regulation.
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