The transcriptional cofactor complex CRSP is required for activity of the enhancer-binding protein Sp1

Activation of gene transcription in metazoans is a multistep process that is triggered by factors that recognize transcriptional enhancer sites in DNA.These factors work with co-activators to direct transcriptional initiation by the RNA polymerase II apparatus [1]. One class of co-activator, the TAF_II subunits of transcription factor TFIID, can serve as targets of activators and as proteins that recognize core promoter sequences necessary for transcription initiation [2-5]. Transcriptional activation by enhancer-binding factors such as Sp1 [6] requires TFIID, but the identity of other necessary cofactors has remained unknown. Here we describe a new human factor, CRSP, that is required together with the TAF_II S for transcriptional activation by Sp1. Purification of CRSP identifies a complex of approximate relative molecular mass 700,000 (M_r [tilde operator] 700K) that contains nine subunits with M_r values ranging from 33K to 200K. Cloning of genes encoding CRSP subunits reveals that CRSP33 is a homologue of the yeast mediator subunit Med7 [7], whereas CRSP150 contains a domain conserved in yeast mediator subunit Rgr1 [8]. CRSP p200 is identical to the nuclear hormone-receptor co-activator subunit TRIP2/PBP [9,10]. CRSPs 34, 77 and 130 are new proteins, but the amino terminus of CRSP70 is homologous to elongation factor TFIIS [11]. Immunodepletion studies confirm that these subunits have an essential cofactor function. The presence of common subunits in distinct cofactor complexes suggests a combinatorial mechanism of co-activator assembly during transcriptional activation.