Some promoters, both in lower and higher organisms (see Ch. 6) function poorly or not at all in the absence of extra proteins known as gene activator proteins, or transcription factors. In addition, there is a class of gene regulator proteins known as repressors that act to turn genes off.
In positive control, an activator is required to turn a gene on, in response to a signal of some kind. In negative control, a gene is switched off by a repressor and is only expressed in the presence of a signal that removes the repressor from the gene. Positive and negative control may be exerted at the level of transcription or at later stages in gene expression. Furthermore, although most activators and repressors are proteins, cases are known in which regulation is due to regulatory RNA or even small molecules.
In both positive and negative control, a small signal molecule, the inducer, typically binds to the regulatory protein and induces gene expression. In the standard model of positive regulation, an inactive activator protein binds the signal molecule and is converted to its DNA-binding form, which then turns on the gene (Fig. 9.09). Similarly, in typical negative regulation, the DNA-binding form of a repressor protein is converted to its inactive form by binding the signal molecule.
inducer A signal molecule that turns on a gene by binding to a regulatory protein negative control or regulation Regulatory mode in which a repressor keeps a gene switched off until it is removed positive control or regulation Control by an activator that promotes gene expression when it binds signal molecule A small molecule that triggers a regulatory response by binding to a regulatory protein
FIGURE 9.09 Principle of Positive and Negative Regulation
In positive regulation, a signal changes the conformation of an inactive regulator, which then becomes active and binds to the regulatory region of a gene. Its presence aids the binding of the RNA poly merase and helps switch on the gene. In negative regulation, a repressor molecule blocks the promoter of the gene. A signal changes the conformation of the repressor, releasing it from the gene and allowing the RNA polymerase to bind.
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