In contrast to the small number of general transcription factors, which are abundant proteins that assemble on the promoters of all genes transcribed by the gene control region for gene X TATA TATA promoter “spacer” DNA coactivatorstranscription regulators general transcription factors RNA polymerase II gene X cis-regulatory sequence gene X Mediator Figure 7–17 The gene control region for a typical eukaryotic gene. the promoter is the DnA sequence where the general transcription factors and the polymerase assemble (see figure 6–15). the cis-regulatory sequences are binding sites for transcription regulators, whose presence on the DnA affects the rate of transcription initiation. these sequences can be located adjacent to the promoter, far upstream of it, or even within introns or entirely downstream of the gene. the broken stretches of DnA signify that the length of DnA between the cis-regulatory sequences and the start of transcription
RNA polymerase II, there are thousands of different transcription regulators devoted to turning individual genes on and off. In eukaryotes, operons—sets of genes transcribed as a unit—are rare, and, instead, each gene is regulated individually. Not surprisingly, the regulation of each gene is different in detail from that of every other gene, and it is difficult to formulate simple rules for gene regulation that apply in every case. We can, however, make some generalizations about how transcription regulators, once bound to gene control regions on DNA, set in motion the series of events that lead to gene activation or repression.
In bacteria, we saw that proteins such as the tryptophan repressor, the Lac repressor, and the CAP protein bind to DNA on their own and directly affect RNA polymerase at the promoter. Eukaryotic transcription regulators, in contrast, usually assemble in groups at their cis-regulatory sequences. Often two or more regulators bind cooperatively, as discussed earlier in the chapter. In addition, a broad class of multisubunit proteins termed coactivators and co-repressors assemble on DNA with them. Typically, these coactivators and co-repressors do not recognize specific DNA sequences themselves; they are brought to those sequences by the transcription regulators. Often the protein–protein interactions between transcription regulators and between regulators and coactivators are too weak for them to assemble in solution; however, the appropriate combination of cis-regulatory sequences can “crystallize” the assembly of these complexes on DNA (Figure 7–18).
As their names imply, coactivators are typically involved in activating transcription and co-repressors in repressing it. In the following sections, we will see that coactivators and co-repressors can act in a variety of different ways to influence transcription after they have been localized on the genome by transcription regulators.
As shown in Figure 7–18, an individual transcription regulator can often participate in more than one type of regulatory complex. A protein might function, varies, sometimes reaching tens of thousands of nucleotide pairs in length. the TATA box is a DnA recognition sequence for the general transcription factor tfiiD. As shown in the lower panel, DnA looping allows transcription regulators bound at any of these positions to interact with the proteins that assemble at the promoter. Many transcription regulators act through Mediator (described in Chapter 6), while some interact with the general transcription factors and rnA polymerase directly. transcription regulators also act by recruiting proteins that alter the chromatin structure of the promoter (not shown, but discussed below).
whereas Mediator and the general transcription factors are the same for all rnA polymerase ii-transcribed genes, the transcription regulators and the locations of their binding sites relative to the promoter differ for each gene.
for example, in one case as part of a complex that activates transcription and in another case as part of a complex that represses transcription. Thus, individual eukaryotic transcription regulators function as regulatory parts that are used to build complexes whose function depends on the final assembly of all of the individual components. Each eukaryotic gene is therefore regulated by a “committee” of proteins, all of which must be present to express the gene at its proper level.
Activator proteins promote the Assembly of rnA polymerase at the Start point of transcription