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Chapter 16: Problem 11

For each of the following types of transcriptional control, indicate whether the protein produced by the regulator gene will be synthesized initially as an active repressor or as an inactive repressor. a. Negative control in a repressible operon b. Negative control in an inducible operon

Short Answer

Expert verified
a. Inactive repressor; b. Active repressor.

Step by step solution

01

Understand Negative Control in a Repressible Operon

In negative control of a repressible operon, transcription occurs naturally unless a co-repressor binds to a repressor protein. The repressor protein is synthesized as inactive initially and becomes active when the co-repressor binds to it, thus halting transcription.
02

Analyze Negative Control in an Inducible Operon

In negative control of an inducible operon, transcription does not occur unless an inducer binds to a repressor protein to deactivate it. Here, the repressor is produced as active initially and prevents transcription until an inducer deactivates it, allowing transcription to proceed.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Repressible Operon
A repressible operon is a fascinating mechanism of gene regulation. It's like having a faucet that remains open under normal conditions, allowing for the continuous flow of gene transcription. In this system, transcription occurs by default until it is turned off, or 'repressed.' This happens when a co-repressor binds to an associated repressor protein.

  • The repressor protein, initially made in an inactive form, doesn't halt transcription on its own.
  • It needs a co-repressor, which is often a product of the operon's pathway, to bind to it.
  • Upon binding, this complex becomes active, attaching to the DNA and blocking further transcription.
This type of feedback system is crucial for saving energy by stopping the production of unnecessary enzymes when they鈥檙e not needed, helping the cell prioritize resources. A classic example is the trp operon in bacteria, which halts its transcription when tryptophan levels are sufficient.
Inducible Operon
Inducible operons work contrary to repressible operons. Picture a light switch that's off until someone turns it on. In these operons, genes remain inactive until they need to be expressed.

  • The regulatory protein is synthesized as an active repressor, naturally preventing transcription.
  • An inducer, often an external molecule, binds to the repressor, causing it to change shape and detach from the DNA.
  • This detachment allows the transcription machinery to move forward and commence the transcription process.
This process allows quick adaptation to environmental changes. For instance, the lac operon in E. coli is a well-known example. It remains off until lactose, an inducer, is present. Lactose's presence signals the need for enzymes to break it down, thus deregulating the operon.
Negative Control
Negative control is a broad term used to describe a regulatory mechanism that inhibits gene transcription via repressor proteins. While there are variations, such as those found in repressible and inducible operons, the main idea remains the same.

  • In both repressible and inducible systems, genetic expression is directly controlled by the presence or absence of repressor activities.
  • With negative control, if the repressor is bound to the DNA, transcription cannot happen.
  • The switch between active and inactive forms is what dictates the repression or activation.
This balance is critical for efficiency, ensuring that the genes are turned on or off as needed in response to internal or external stimuli. Negative control serves as a vital mechanism in maintaining homeostasis within the cell, preventing overproduction of proteins in stable or hostile environments.

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Most popular questions from this chapter

Briefly describe the lac operon and how it controls the metabolism of lactose.

A mutant strain of \(E .\) coli produces \(\beta\) -galactosidase in both the presence and the absence of lactose. Where in the operon might the mutation in this strain be located?

A mutation at the operator prevents the regulator protein from binding. What effect will this mutation have in the following types of operons? a. Regulator protein is a repressor of a repressible operon. b. Regulator protein is a repressor of an inducible operon.

Listed in parts \(a\) through \(g\) are some mutations that were found in the \(5^{\prime}\) UTR of the trp operon of \(E .\) coli. What will the most likely effect of each of these mutations be on the transcription of the trp structural genes? a. A mutation that prevents the binding of the ribosome to the \(5^{\prime}\) end of the mRNA \(5^{\prime}\) UTR b. A mutation that changes the Trp codons in region 1 of the mRNA \(5^{\prime}\) UTR into codons for alanine c. A mutation that creates a stop codon early in region 1 of the mRNA \(5^{\prime}\) UTR d. Deletions in region 2 of the mRNA \(5^{\prime}\) UTR e. Deletions in region 3 of the mRNA \(5^{\prime}\) UTR f. Deletions in region 4 of the mRNA \(5^{\prime}\) UTR g. Deletion of the string of adenine nucleotides that follows region 4 in the \(\overline{5}^{\prime}\) UTR

Transformation is a process in which bacteria take up new DNA released by dead cells and integrate it into their own genomes (see p. 265 in Chapter 9 ). In Streptococcus pneumoniae (which causes many cases of pneumonia, inner-ear infections, and meningitis), the ability to carry out transformation requires from 105 to 124 genes, collectively termed the com regulon. The com regulon is activated in response to a protein called competence-stimulating peptide (CSP), which is produced by the bacteria and exported into the surrounding medium. When enough CSP accumulates, it attaches to a receptor on the bacterial cell membrane, which then activates a regulator protein that stimulates the transcription of genes within the com regulon and sets in motion a series of reactions that ultimately result in transformation. Does the com regulon in Streptococcus pneumoniae exhibit positive or negative control? Explain your answer.
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