Transcription cofactors mediate access to genes in chromatin, they help to establish, maintain or activate regulatory networks and they affect the formation and activity of basal initiation complexes. In yeast a large multiprotein complex called SRB/Mediator complex has been identified based on its ability to enhance basal and to facilitate activated transcription (Flanagan et al., 1991; Kim et al., 1994; Koleske and Young, 1994; Lee et al., 1999; Myers and Kornberg, 2000; Yudkovsky et al., 2000). Independently, several mammalian Mediator activities were discovered that supported (TRAP/SMCC, ARC, DRIP, human Srb/Mediator) or repressed (NAT) specifically the function of activators (Hampsey and Reinberg, 1999; Lemon and Tjian, 2000; Malik and Roeder, 2000). Purification and cloning of the individual polypeptides suggested that TRAP (Fondell et al., 1999; Ito et al., 1999), SMCC (Gu et al., 1999), ARC (Näar et al., 1999), DRIP (Rachez et al., 1999), NAT (Sun et al., 1998) and hSrb/Mediator (Boyer et al., 1999) are highly related, sharing many subunits. CRSP (Ryu et al., 1999) and PC2, a component of the coactivator fraction USA (Meisterernst et al., 1991), were shown to contain a limited subset of Mediator components (Malik et al., 2000). To date the individual role and the interplay of the variant Mediator forms is not well understood. TRAP/SMCC binds to several activation domains and like PC2 stimulates activator-dependent transcription in vitro in conjunction with other cofactors (Fondell et al., 1999; Ito et al., 1999; Malik et al., 2000). ARC and DRIP also bind to specific activators but enhance their effects exclusively in chromatin (Näar et al., 1999). ARC binds the coactivator CBP and contains one specific component termed ARC105/TIG-1 that appears to be absent in the other Mediator complexes (Näar et al., 1999). There are other differences in composition between the different forms, whose significance and functional importance remain elusive (Malik and Roeder, 2000).
Previous studies in mammals describe Mediator as a complex that is required for the function of regulatory activators. Here we report a critical general and basal function of human Mediator. An immunoprecipitation-based purification strategy led to the identification of two distinct Mediator forms, only one of which serves as a general RNA polymerase II transcription factor. The novel Mediator function is dependent on the context of crude physiological transcription systems.