BIOCHEMICAL and genetic studies suggest the existence of mediators that work between the activation domains (ADs) of regulatory proteins and the basic transcriptional machinery. We have previously shown genetically that Sugl interacts with the AD of the yeast activator Ga141. Here we provide evidence that the Sugl protein of yeast binds directly to the ADs of Gal4 and the viral activator, VP16. Sugl protein is associated with the TATA-binding protein in vivo and binds to it in vitro, consistent with a mediator function. We also demonstrate that Sugl is not a component of the 26S proteasome, contrary to previous reports2-5. Sugl is a member of a large, highly conserved family of ATPases, implying a role for ATP hydrolysis in the activation of transcription.
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