The mediator is an approximately 20 protein complex that is essential for the transcription of most genes in yeast. It is contacted by a number of gene-specific activators, but the details of these interactions are not well understood in most cases. Here, evidence is presented that the mediator component Ga111 represents at least one target of the Ga14 activation domain (AD). Deletion of Ga111 is shown to decrease the affinity of the Ga14 AD for the mediator, and direct binding of an N-terminal domain of Ga111 with the Ga14 AD is demonstrated. Quantitative studies, however, indicate that the KD of the 1:1 Ga14 AD - Ga111 complex is modest. Combined with in vivo data showing that Δga111 cells exhibit reduced, but still significant, Ga14-mediated gene expression, these results suggest that the dimeric activator might also contact another protein in the mediator in addition to Ga111.
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