Consistent with the complexity of the temporally regulated processes that must occur for growth and development of higher eukaryotes, it is now apparent that transcription is regulated by the formation of multicomponent complexes that assemble on the promoters of genes. These complexes can include (in addition to the five or more general transcription factors and RNA polymerase 11) DNA-binding proteins, transcriptional activators, coactivators, adaptors and various accessory proteins. The best studied example of a complex that includes a transcriptional adaptor, accessory proteins and a DNA-binding protein is that involving the herpes simplex virus VP16 protein. Evidence suggests that the adenovirus Ela protein and the cellular Spl and CTF/NFl transcription factors also function through adaptors or coactivators. Each additional component of the transcription complex provides the cell with another point at which to exert control of gene expression.
General Components of Transcriptional Activation
Current views of transcriptional activation by RNA polymerase I1 are summarized in the diagram (Fig. ). Two regions of the promoter DNA are important. A region in the vicinity of the transcription start site, termed a corc promoter, confoims to the TATA consensus motif. This region of DNA is bound by the general transcription factors that are important for both basal and activated levels of transcription of essentially all RNA polymerase TI transcribed genes (for a review see ref. ). The general transcription factors include TFIIA, TFITB, TFIID (which binds directly to the TATA box), TFIIE, TFIIF (RAP30/74) and probably additional protein^(^-^). DNA elements generally located upstream of the core promoter are responsible for gene-specific stimulation of the level of transcription. These sequence elements are tcrmed either promoter-proximal elements or enhancers, depending on their proximity to the transcriptional start site. and conform to various consensus sequences that bind to sequence-spccific transcription factors.
Sequence-specific transcription factors arc comprised of two critical regions, typically located on separable domains (for a review, see ref. 6). A DNA-binding