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 todirect transcriptional initiation by the RNA polymerase II apparatus1. One class of co-activator, the TAFII subunits of transcription factor TFIID, can serve as targets of activators and as proteins that recognize core promoter sequences necessary fortranscription initiation2,3,4,5. Transcriptional activation by enhancer-binding factors such as Sp1 (ref. 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 TAFIIs for transcriptional activation by Sp1. Purification of CRSP identifies a complex of approximate relative molecular mass 700,000 (Mr ∼700K) that contains nine subunits with Mr values ranging from 33K to 200K. Cloning of genes encoding CRSP subunits reveals that CRSP33 is a homologue of the yeast mediator subunit Med7 (ref. 7), whereas CRSP150 contains a domain conserved in yeast mediator subunit Rgr1 (ref. 8). CRSP p200 is identical to the nuclear hormone-receptor co-activator subunit TRIP2/PBP9,10. CRSPs 34, 77 and 130 are new proteins, but the amino terminus of CRSP70 is homologous to elongation factor TFIIS11. 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.