An enormous body of work generated over the past three decades has revealed that eukaryotic gene transcription is a remarkably intricate biochemical process that is tightly regulated at many levels. Biochemical and genetic analysis of various model organisms has identified an astounding number of protein factors responsible for transcriptional control. Although a large assortment of gene-specific DNA-binding regulators was somewhat anticipated, the sheer complexity of the general machinery relative to prokaryotes has been a surprise. Even more unexpected were the numerous and intricate layers of control imposed by the diversification of co-activators and co-repressors, some of which possess enzymatic activities. Many interactions between the identified factors and some of their rate-limiting steps have been discerned. Despite these advances, surprisingly little is known about the detailed mechanisms by which individual genes are turned on or off in a cell. Recent evidence suggests that there is an ordered progression of events leading to RNA synthesis in vivo and that a highly structured eukaryotic nucleus may be important in orchestrating transcription. In this review, we present our interpretation of recent findings and discuss various models that integrate these observations with the emerging elaborate molecular apparatus that has evolved to control gene expression.

Nuclear receptors modulate the transcription of genes in direct response to small lipophilic ligands. Binding to ligands induces conformational changes in the nuclear receptors that enable the receptors to interact with several types of cofactor that are critical for transcription activation (transactivation). We previously described a distinct set of ligand-dependent proteins called DRIPs, which interact with the vitamin D receptor (VDR); together, these proteins constitute a new cofactor complex. DRIPs bind to several nuclear receptors and mediate ligand-dependent enhancement of transcription by VDR and the thyroid-hormone receptor in cell-free transcription assays. Here we report the identities of thirteen DRIPs that constitute this complex, and show that the complex has a central function in hormone-dependent transactivation by VDR on chromatin templates. The DRIPs are almost indistinguishable from components of another new cofactor complex called ARC, which is recruited by other types of transcription activators to mediate transactivation on chromatin-assembled templates. Several DRIP/ARC subunits are also components of other potentially related cofactors, such as CRSP, NAT, SMCC and the mouse Mediator, indicating that unique classes of activators may share common sets or subsets of cofactors. The role of nuclear-receptor ligands may, in part, be to recruit such a cofactor complex to the receptor and, in doing so, to enhance transcription of target genes.

An array of regulatory protein and multi-subunit cofactors has been identified that directs eukaryotic gene transcription. However, establishing the specific functions of various related cofactors has been difficult owing to the limitations inherent in assaying transcription in animals and cells indirectly. Here we describe, using an integrated chromatin-dependent reconstituted transcription reaction, the purification and identification of a multi-subunit cofactor (PBAF) that is necessary for ligand-dependent transactivation by nuclear hormone receptors. A highly related cofactor, human SWI/SNF, and the ISWI-containing chromatin-remodelling complex ACF both fail to potentiate transcription. We also show that transcriptional activation mediated by nuclear hormone receptors requires TATA-binding protein (TBP)-associated factors (TAFs) as well as the multi-subunit cofactors ARC/CRSP. These studies demonstrate functional selectivity amongst highly related complexes involved in gene regulation and help define a more complete set of factors and cofactors required to activate transcription.