Regulation of transcription initiation is of fundamental importance in controlling gene expression. In many cases, the availability of factors that bind to specific sites in promoters is the switch that turns a gene on. Various strategies to control the binding of specific factors have been discovered. One of the most straight forward is de novo synthesis of the specific factor. This requires an additional level of transcription regulation and translation of the mRNA that encodes the specific factor. All of these steps take some time; therefore, this regulatory scheme is not used in situations in which rapid responses are required. Instead, it is used more commonly in regulating developmental pathways.
Several mechanisms are used for rapid regulation of the activity of existing transcription factors. One mechanism involves the formation of an active factor from two inactive subunits. This association can be regulated through synthesis or by modification of preexisting subunits, leading to their association. Binding of small-molecule ligands is another means of controlling transcription factor activity. In this case, the binding of the ligand induces a .conformational change that leads to DNA binding and transcription activation Interaction of transcription factors with inhibitory subunits is also used to regulate factor activity. In this case, the binding of the ligand induces a conformational change that leads to DNA binding and transcription activation. Interaction of transcription factors with inhibitory subunits is also used to regulate factor activity. The DNA binding or activation potential is held in check until the appropriate signal leads to dissociation of the inhibitory factor. Covalent modification for example, by phosphorylation is also used to convert inactive transcription factors to a functional form. Finally, the ability of transcription factors to bind DNA may be regulated by restricting their localization to the cytoplasm. These regulatory schemes are not mutually exclusive, and many regulatory pathways employ several different levels of regulation.