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Chapter 18: Problem 4

Briefly describe two different ways in which intragenic suppressors can reverse the effects of mutations.

Short Answer

Expert verified
Intragenic suppressors can restore protein structure or re-establish active site function to counteract initial mutations.

Step by step solution

01

Understand Intragenic Suppressors

Intragenic suppressors are second mutations that occur within the same gene as the initial mutation, which counteract or neutralize the effects of the original mutation. They restore the function that was lost or affected due to the original mutation.
02

Mechanism One - Restoration of Protein Structure

One way intragenic suppressors work is by restoring the original protein structure that was disrupted by the first mutation. For instance, if a mutation led to an amino acid substitution that destabilized the protein structure, a suppressor mutation might occur at another site on the same protein, leading to a substitution that compensates for the initial structural change, stabilizing the protein.
03

Mechanism Two - Re-establishment of Active Site Function

Another way is through the re-establishment of function at the protein's active site. If the first mutation altered the active site, reducing the protein's functionality, a second mutation might modify another site away from the active site, altering the conformation of the protein to re-enable activity at the active site despite the original mutation.

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Key Concepts

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

Mutation Reversal
Mutation reversal refers to the process by which the effects of a deleterious mutation are neutralized. Intragenic suppressors are an intriguing way this can occur within the same gene. These suppressors introduce a second mutation that acts as a buffer against the damaging effects of the initial mutation. This results in a restoration of function without removing the original mutation itself.

In some instances, the first mutation may have caused a significant change in the protein's behavior or function. However, with a carefully positioned intragenic suppressor, this change can be counteracted. Imagine it like a biological "undo" button that mitigates the impact of a mistake. This form of mutation reversal can help bring back the normal functioning of the gene product, making it behave as though the initial mutation never occurred.
  • Intragenic suppressors arise within the same gene as the initial mutation.
  • They can stabilize the gene鈥檚 product, despite the presence of the original mutation.
  • Serve as a natural method of 鈥榬epairing鈥� genetic errors.
Protein Structure Restoration
Protein structure restoration is a vital mechanism by which intragenic suppressors alleviate the consequences of a mutation. Proteins rely heavily on their three-dimensional structures to function properly. If a mutation alters this structure, the protein may become unstable or inactive. That's where intragenic suppressors can play a role.

These suppressors can lead to additional changes in the protein that restore its original shape or structure. For example, if an initial mutation substituted an amino acid that weakened the protein's fold, a new mutation might introduce another amino acid substitution that restores the stability. This restoration ensures that the protein retains its proper configuration and continues to function.
  • The suppressor mutation acts to counter or adjust changes in protein shape.
  • Successful suppression leads to a protein folding back into its functional form.
  • Helps maintain protein integrity despite genetic alterations.
Active Site Re-establishment
The active site of a protein is crucial for its activity, often being the specific region where chemical reactions occur. A mutation in the active site can render a protein nonfunctional. However, an intragenic suppressor may assist in re-establishing its activity even if it doesn鈥檛 directly occur in the active site.

Sometimes, a suppressor mutation elsewhere in the protein's sequence can alter its overall conformation, indirectly restoring the functionality of the active site. By changing the way a protein folds or by adjusting its shape, the suppressor mutation can compensate for the initial change at or near the active site. This enables the protein to perform its intended biological roles effectively.
  • Active site re-establishment focuses on restoring what the mutation disrupted.
  • Suppression can act from sites distant from the active site itself.
  • Maintains the enzyme鈥檚 or protein鈥檚 biological activity.

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Most popular questions from this chapter

Tay-Sachs disease is a severe autosomal recessive genetic disease that produces deafness, blindness, seizures, and, eventually, death at 2 to 3 years of age. The disease results from a defect in the HEXA gene, which encodes hexosaminidase A. This enzyme normally degrades \(G_{\mathrm{M} 2}\) gangliosides. In the absence of hexosaminidase A, G_M2 gangliosides accumulate in the brain. The results of molecular studies showed that the most common mutation causing Tay-Sachs disease is a 4 -bp insertion that produces a downstream premature stop codon. Results of further studies have revealed that the transcription of the HEXA gene is normal in people who have Tay-Sachs disease, but the HEXA mRNA is unstable. Propose a mechanism to account for how a premature stop codon could cause mRNA instability.

What is the difference between a transition and a transversion? Which type of base substitution is more common?

In many eukaryotic organisms, a significant proportion of cytosine bases are naturally methylated to 5 -methylcytosine. Through evolutionary time, the proportion of AT base pairs in the DNA of these organisms increases. Can you suggest a possible mechanism for this increase?

How do alkylating agents, nitrous acid, and hydroxylamine produce mutations?

Robert Bost and Richard Cribbs studied a strain of \(E .\) coli (araB14) that possessed a nonsense mutation in the structural gene that encodes Lribulokinase, an enzyme that allows the bacteria to metabolize the sugar arabinose (R. Bost and R. Cribbs. 1969. Genetics 62:1-8). From the araB14 strain, they isolated some bacteria that possessed mutations that caused them to revert back to the wild type. Genetic analysis of these revertants showed that they possessed two different suppressor mutations. One suppressor mutation ( \(R 1\) ) was linked to the original mutation in L-ribulokinase and probably occurred at the same locus. By itself, this mutation allowed the production of L-ribulokinase, but the enzyme produced was not as effective in metabolizing arabinose as the enzyme encoded by the wild-type allele. The second suppressor mutation \(\left(S u^{\mathrm{B}}\right)\) was not linked to the original mutation. In conjunction with the \(R 1\) mutation, \(S u^{\mathrm{B}}\) allowed the production of L-ribulokinase, but \(S u_{\mathrm{B}}\) by itself was not able to suppress the original mutation. a. On the basis of this information, are the \(R 1\) and \(S u^{\mathrm{B}}\) mutations intragenic suppressors or intergenic suppressors? Explain your reasoning. b. Propose an explanation for how \(R 1\) and \(S u^{\mathrm{B}}\) restore the ability of araB14 to metabolize arabinose and why \(S u^{\mathrm{B}}\) is able to more fully restore this ability.
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