Biochemical pathway

Precursor X

17 In some operons, product molecules may, in turn, bind to the regulator protein to either activate it or turn it off.

Intermediate products

Product Y

which a regulatory protein acts as an activator, stimulating transcription. In the next sections, we will consider several varieties of these two basic control mechanisms.

Negative inducible operons In an operon with negative control at the operator site, the regulatory protein is a repressor—the binding of the regulator protein to the operator inhibits transcription. In a negative inducible operon, transcription and translation of the regulator gene produce an active repressor that readily binds to the operator (< Figure 16.4a). Because the operator site overlaps with the promoter site, the binding of this protein to the operator physically blocks the binding of RNA polymerase to the promoter and prevents transcription. For transcription to take place, something must happen to prevent the binding of the repressor at the operator site. This type of system is said to be inducible, because transcription is normally off (inhibited) and must be turned on (induced).

Transcription is turned on when a small molecule, an inducer, binds to the repressor. < Figure 16.4b shows that, when precursor V (acting as the inducer) binds to the repressor, the repressor can no longer bind to the operator. Regulatory proteins frequently have two binding sites: one that binds to DNA and another that binds to a small molecule such as an inducer. Binding of the inducer alters the shape of the repressor, preventing it from binding to DNA. Proteins of this type, which change shape on binding to another molecule, are called allosteric proteins.

When the inducer is absent, the repressor binds to the operator, the structural genes are not transcribed, and enzymes D, E, and F (which metabolize precursor V) are not synthesized (see Figure 16.4a). This is an adaptive mechanism: because no precursor V is available, it would be wasteful for the cell to synthesize the enzymes when they have no substrate to metabolize. As soon as precursor V becomes available, some of it binds to the repressor, rendering the repressor inactive and unable to bind to the operator site. Now RNA polymerase can bind to the promoter and transcribe the structural genes. The resulting mRNA is then translated into enzymes D, E, and F, which convert substrate V into product W (see Figure 16.4b). So, an operon with negative inducible control regulates the synthesis of the enzymes economically: the enzymes are synthesized only when their substrate (V) is available.

Negative repressible operons Some operons with negative control are repressible, meaning that transcription normally takes place and must be turned off, or repressed. The regulator protein in this type of operon also is a repres-sor but is synthesized in an inactive form that cannot by itself bind to the operator. Because there is no repressor bound to the operator, RNA polymerase readily binds to the promoter and transcription of the structural genes takes place (Figure 16.5a).

To turn transcription off, something must happen to make the repressor active. A small molecule called a core-

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