Mediating activation

Abstract

The yeast SRB/MED complex is tightly associated with RNA polymerase II (RNAPII) and mediates transcriptional activation in vitro and in vivo. Given the conservation in structure and function among components of the eukaryotic RNAPII transcriptional machinery, it seemed reasonable that human counterparts of the yeast SRB/MED complex would exist. Indeed, homologs of yeast Srb7, Srb10 and Srb11 have been identified as components of human RNAPII holoenzyme preparations, and more recently as components of mammalian complexes that are free of RNAPII. Based on affinity purification of the human SRB proteins, Gu et al.1 now report a novel complex of 1.5 MDa, denoted SMCC, that includes homologs of the Srb7, Srb10, Srb11, Med6, Med10 and Rgr1 subunits of the SRB/MED complex. Moreover, SMCC turns out to be identical to TRAP, a complex initially identified as a mediator of activation by a human thyroid hormone receptor (TR) complex2. In a separate study designed to identify the requirements for activation by Sp1, Ryu et al.3 purified a 0.7 MDa complex, denoted CRSP, that includes a homolog of yeast Med7 and an Rgr1-related protein. The ability of CRSP to mediate activation by Sp1 is dependent upon the TAF subunits of TFIID, but is independent of other transcriptional cofactors. In contrast, SMCC is dependent upon the cofactor PC4 and upon limiting concentrations of TFIIH for activation. These cofactor requirements are noteworthy because a hallmark of the SRB/MED complex is its ability to function independently of other cofactors, including TAFs. However, in an effort to define the requirements for TR-mediated activation, Fondell et al.4 demonstrated that the TAF-subunits of TFIID are dispensable for high levels of activation, requiring only PC2, PC4 and the SMCC (TRAP) complex as cofactors. This is a surprising result given that TAFs were considered to be a universal requirement for activation in purified metazoan cell-free systems, yet is consistent with an earlier study from the Roeder laboratory demonstrating activation in a HeLa nuclear extract immunodepleted of TAFs. These results suggest that SMCC and TAFs are functionally redundant, at least with respect to TR-mediated activation in vitro. On the other hand, the different subunit compositions and cofactor requirements for activation mediated by SMCC and CRSP indicate that these complexes are not redundant. However, CRSP might be a subcomplex of SMCC, a premise suggested by the organization of the SRB/MED complex into structurally and functionally distinct domains. Taken together, these results imply that different activators utilize specific mediator subunits, either as components of a common mediator complex or by mixing and matching subunits to generate activator-specific complexes. Accordingly, a key role for transcriptional activators would be to recruit specific mediator complexes to the promoter.