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Chapter 16: Problem 21

A bacterial operon is responsible for the production of the biosynthetic enzymes needed to make the hypothetical amino acid tisophane (tis). The operon is regulated by a separate gene, \(R,\) deletion of which causes the loss of enzyme synthesis. In the wild-type condition, when tis is present, no enzymes are made; in the absence of tis, the enzymes are made. Mutations in the operator gene (O) result in repression regardless of the presence of tis. Is the operon under positive or negative control? Propose a model for (a) repression of the genes in the presence of tis in wild-type cells and (b) the mutations.

Short Answer

Expert verified
Answer: The bacterial operon is under negative control. In the presence of tisophane in wild-type cells, enzyme synthesis is inhibited as tisophane binds to the repressor protein, increasing its affinity for the operator. In mutated operator gene (O), the operator is always bound by the repressor, causing constitutive repression of enzyme synthesis, irrespective of the presence or absence of tisophane.

Step by step solution

01

Understanding of the wild-type situation

In the wild-type condition, the bacterial operon responds to the presence or absence of tisophane. When tis is present, no enzymes are made, while in the absence of tis, enzymes are synthesized. This indicates that the synthesis of enzymes is inhibited or repressed when tisophane is present. Thus, the control mechanism appears to be a negative one, as the presence of the substance (tis) hinders the process (enzyme synthesis).
02

Analyzing the operon in the presence of tisophane in wild-type cells

In the wild-type cells, the presence of tisophane seems to inhibit the expression of the genes responsible for enzyme synthesis. Since the operon is under negative control, we can propose a model in which tisophane binds to the repressor protein (R) encoded by gene \(R\). This binding results in increased affinity of the repressor for the operator site (O), which prevents the transcription of the enzymes needed for tis synthesis.
03

Analyzing the mutations in the operator gene (O)

Mutations in the operator gene (O) result in repression of enzyme synthesis, regardless of the presence of tisophane. This indicates that the mutated operator gene is insensitive to the repression/release mechanism by the repressor (R) since it is always bound by the repressor. The mutated operator site can no longer distinguish between the presence or absence of tisophane, leading to a constitutive repression of enzyme synthesis.
04

Conclusion:

Based on the given information, we can determine that the bacterial operon responsible for the enzyme synthesis required for tisophane production is under negative control. In the presence of tisophane in wild-type cells, the repressor binds to tisophane, increasing its affinity for the operator, and inhibiting enzyme synthesis. In mutated operator gene (O), the operator is always bound by the repressor, causing constitutive repression of enzyme synthesis, irrespective of the presence or absence of tisophane.

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Most popular questions from this chapter

The locations of numerous \(\operatorname{lac} I\) and \(\operatorname{lac} I^{\mathrm{S}}\) mutations have been determined within the DNA sequence of the lacI gene. Among these, \(l a c I\) mutations were found to occur in the \(5^{\prime}\) -upstream region of the gene, while \(\operatorname{lac} I^{\mathrm{S}}\) mutations were found to occur farther downstream in the gene. Are the locations of the two types of mutations within the gene consistent with what is known about the function of the repressor that is the product of the lacI gene?

Contrast the role of the repressor in an inducible system and in a repressible system.

Both attenuation of the \(t r p\) operon in \(E\). coli and riboswitches in B. subtilis rely on changes in the secondary structure of the leader regions of mRNA to regulate gene expression. Compare and contrast the specific mechanisms in these two types of regulation.

Describe the experimental rationale that allowed the lac repressor to be isolated.

Keeping in mind the life cycle of bacteriophages discussed earlier in the text (see Chapter 6 ), consider the following problem: During the reproductive cycle of a temperate bacteriophage, the viral DNA inserts into the bacterial chromosome where the resultant prophage behaves much like a Trojan horse. It can remain quiescent, or it can become lytic and initiate a burst of progeny viruses. Several operons maintain the prophage state by interacting with a repressor that keeps the lytic cycle in check. Insults (ultraviolet light, for example) to the bacterial cell lead to a partial breakdown of the repressor, which in turn causes the production of enzymes involved in the lytic cycle. As stated in this simple form, would you consider this system of regulation to be operating under positive or negative control?
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