FIGURE 6.13 Principle of Negative Regulation by a Repressor
The LacI protein is bound to the operator site, within the promoter region of a gene that affects lactose metabolism. The inducer binds to LacI, changing its conformation and causing its release from the DNA. The RNA poly merase is then free to transcribe the gene.
expression is known as the inducer. The result of this is that the genes intended for using maltose are only induced when this particular sugar is available. The same general principle applies to most nutrients, although the details of the regulation may vary from case to case.
Repressors are proteins that switch genes off.
Negative Regulation Results from the Action of Repressors
Genes may be controlled by positive or negative regulation. In positive regulation, an activator protein binds to the DNA only when the gene is to be turned on. In negative regulation, a repressor protein binds to the DNA and insures that the gene is turned off. Only when the repressor is removed from the DNA can the gene be transcribed. The site where a repressor binds is called the operator sequence. Like activator proteins, repressor proteins alternate between DNA-binding and nonbinding forms. In this case, binding of the inducer to the repressor causes it to change from its DNA-binding form to the nonbinding form.
Historically, negative regulators were discovered before activators. The best known example is the lactose repressor, the LacI protein (Fig. 6.13). Lactose is another sugar, found in milk, on which bacteria such as E. coli can grow. When no lactose is available, the LacI protein binds to its operator sequence, which overlaps part of the promoter and the front part of the coding region for the genes for using lactose. When lactose is present, the LacI protein changes shape and is released from the DNA and the lactose genes are induced. Overall, the result is the same as for maltose: when lactose is available, the genes for using it are switched on and when there is no lactose, the genes are turned off.
inducer Small signal molecule that binds to a regulatory protein and thereby causes a gene to be switched on LacI protein Repressor that controls the lac operon negative regulation Regulatory mode in which a repressor keeps a gene switched off until it is removed operator Site on DNA to which a repressor protein binds positive regulation Control by an activator that promotes gene expression when it binds repressor Regulatory protein that prevents a gene from being transcribed
FIGURE 6.14 Allosteric Protein Binds a Signal Molecule and Changes Shape
The two subunits shown have a signal-binding site and a DNA-binding site. When the signal molecule binds to the subunits, they pair and change conformation They are then able to bind to DNA.
Signal binding site
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