hSrb7, an essential human Mediator component, acts as a coactivator for the thyroid hormone receptor

Nuclear hormone receptors interact with the basal-transcriptional complex and/or coactivators to regulate transcriptional activation. These activator-target interactions recruit the transcriptional machinery to the promoter and may also stimulate transcriptional events subsequent to the binding of the machinery to the promoter or enhancer element. We describe a novel functional interaction of the nuclear thyroid receptor (TR), with a human Mediator component (hSrb7), and a human TFIIH component (hMo15). In mammalian two-hybrid experiments as well as in GST-pull down assays, hSrb7 interacts with TR but not with other nuclear receptors such as the retinoic acid receptor (RAR) or the vitamin D receptor (VDR). Whereas hMo15 also interacts with VDR and RAR in mammalian two-hybrid assays, no association of hSrb7 with VDR or RAR is found. Accordingly, cotransfection of TR and hSrb7 increases thyroid hormone (T3)-dependent transcription in an AF-2-dependent manner, while hSrb7 causes no stimulation of vitamin D- or retinoic acid-mediated transactivation. These results reveal a novel co-activator role for hSrb7 and hMo15 on TR transcriptional responses, and demonstrate that different receptors can selectively target different co-activators or general transcription factors to stimulate transcription.