Which of the following is true regarding positive inducible operons?
a) Inducer molecules bind to the activator, preventing transcription
b) Operators bind to DNA, stimulating transcription
c) Inducer molecules bind to the operator, activating transcription
d) Inducer molecules bind to activator, stimulating transcription
D is correct. A positive inducible operon is an operon that is usually inactive under normal conditions. In this way, the activator protein usually cannot bind to the DNA to stimulate transcription. When an inducer molecule is present, it will bind to the activator protein, causing a conformational change that allows the activator protein to bind to the DNA to stimulate transcription.
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In prokaryotes, as in other organisms, gene expression is tightly regulated. Structural proteins that have related functions are encoded together within the genome in various blocks called operons. An operon is a DNA sequence that includes a promoter, an operator, and the genes that are regulated together. The promoter is a DNA sequence that is recognized by RNA polymerase for transcription. The operator is a DNA sequence that a repressor can bind to, and repressors are proteins that can either allow or inhibit transcription. The set of genes in an operon are transcribed together by way of the promoter, creating a polycistronic transcript. In other words, multiple proteins are encoded by a single mRNA transcript. This form of gene expression is actually quite efficient. Instead of producing separate mRNAs for each protein required for a biochemical process, prokaryotes instead produce a single mRNA encoding all the proteins necessary for that process.
Prokaryotic gene expression can be controlled in two ways: positive control and negative control. Negative control was discussed in a previous post. This post will cover positive control of prokaryotic gene expression.
In positive control, an activator protein binds to DNA to stimulate transcription. The activator protein usually binds to a site on the DNA that is separate from the operator protein. The activator protein then is essentially the opposite of a repressor protein used in negative control of prokaryotic gene expression. Remember that when a repressor protein binds to DNA, it prevents transcription. When an activator protein binds, it activates or enhances transcription. Also, similar to repressor proteins, activator proteins can bind to inducer molecules. When this happens, it causes a conformational change in the activator protein, either allowing or preventing the activator protein from binding to DNA. If binding between the activator and the inducer enables transcription, the operon is said to be positive inducible. If the same binding inhibits transcription, the operon is said to be positive repressible.
Positive inducible operons are operons whose expression is usually inactive. However, the presence of an inducer molecule can turn on the transcription of this type of operon. Figure 1 shows how a positive inducible operon works. Under normal conditions, the activator protein cannot bind to DNA to stimulate transcription. At first then, transcription does not occur. However, when an inducer molecule is present, the inducer molecule binds to the activator protein. This process causes a conformational change in the activator protein that allows it to bind to the DNA to stimulate transcription.
Unlike positive inducible operons, positive repressible operons usually have transcription activated in their normal state. However, the presence of an inducer molecule can prevent or inhibit transcription from occurring. Figure 2 shows a positive repressible operon. Under normal conditions, the activator protein will bind to DNA, which will stimulate the production of mRNA molecules. However, when an inducer molecule is present, the inducer molecule will bind to the activator protein. This process prevents the activator protein from binding the DNA and stops transcription.
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