The multi-subunit general transcription apparatus: identification of tissue-specific and gene-selective subunits. Diversified metazoan transcription initiation complexes. a, The eukaryotic transcriptional apparatus can be subdivided into three broad classes of multi-subunit ensembles that include the RNA polymerase II core complex and associated general transcription factors (TFIIA, -B,-D,-E,-F and -H), multi-subunit cofactors (mediator, CRSP, TRAP, ARC/DRIP, and so on) and various chromatin modifying or remodelling complexes (SWI/SNF, PBAF, ACF, NURF and RSF). b, c, Metazoan organisms have evolved multiple gene-selective and tissue-specific TFIID-like assemblies by using alternative TAFs (TBP-associated factors such as the ovarian-specific TAF105) as well as TRFs (TBP-related factors such as TRF2 in Drosophila and mice) to mediate the formation of specialized RNA polymerase initiation complexes that direct the transcription of tissue-specific and gene-selective programmes of expression.