Advanced Microbiology Lecture 12

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Questions and Answers

What role does a co-repressor play in the function of a repressor protein?

It prevents the repressor from binding to the operator.

It activates the repressor in the presence of the end product.

It allows the repressor to bind to the operator site, decreasing gene expression. (correct)

It stabilizes the mRNA produced by the operon.

What is the primary function of catabolic operons?

To synthesize compounds needed for cell function.

To regulate the expression of regulatory proteins.

To inhibit the activity of biosynthetic pathways.

To degrade compounds in order to obtain catabolites for metabolism. (correct)

In the E.coli ara operon, what happens when arabinose is absent?

AraC P2 activates transcription.

AraC P1 binds to araI1 and araI2 to promote operon expression.

AraC P1 functions as an anti-activator by binding to araO2 and araI1. (correct)

AraC is unable to bind any sites, leading to increased transcription.

What does the term 'aporepressor' refer to?

A repressor that requires a co-repressor to be active. Signup and view all the answers

Which of the following statements describes negative regulation of operons?

Genes are expressed constantly until actively turned off by a repressor. Signup and view all the answers

What distinguishes positive regulation in the context of operons from negative regulation?

It requires an activator protein for gene transcription. Signup and view all the answers

Which of the following accurately describes the function of AraC in the ara operon?

AraC possesses two protein states that regulate the operon based on arabinose presence. Signup and view all the answers

What is the primary purpose of biosynthetic operons?

To produce compounds necessary for the cell, such as amino acids. Signup and view all the answers

What is the effect on gene expression when a repressor is bound to the operator site?

Gene expression is decreased or blocked. Signup and view all the answers

What is the primary role of repressors in transcriptional regulation?

To bind to the operator and inhibit the initiation of transcription Signup and view all the answers

Which of the following statements about transcriptional regulators is true?

Both activators and repressors can function as regulatory proteins. Signup and view all the answers

In a repressible system, how is gene expression typically controlled?

Gene expression is reduced when the end product is present in excess. Signup and view all the answers

What defines a constitutive mutant?

It has a mutation that prevents transcriptional regulation. Signup and view all the answers

What is the function of an inducer in gene expression regulation?

To activate an activator or inactivate a repressor, increasing gene expression. Signup and view all the answers

How do activators contribute to the process of transcription?

By stabilizing RNA polymerase binding to the promoter. Signup and view all the answers

What mechanism does a repressor use to inhibit transcription initiation?

Blocking RNA polymerase from binding or proceeding at the promoter. Signup and view all the answers

Which term describes the regulation of gene expression that occurs after the transcription process?

Posttranscriptional regulation Signup and view all the answers

What is the role of co-repressors in gene regulation?

They decrease gene expression by enabling repressor function. Signup and view all the answers

Which motif is mentioned as relevant for transcriptional regulation?

Helix-Turn-Helix motifs Signup and view all the answers

Study Notes

Advanced Microbiology: Lecture 12 - Gene Expression Regulation

Gene expression is controlled to ensure cells only express necessary genes in specific environments.
Gene expression regulation occurs at various levels: transcriptional, post-transcriptional, translational, and post-translational.
Most transcriptional regulation happens at the promoter region using proteins called transcriptional regulators.
Repressors bind to DNA operator sequences, preventing transcription initiation (negative regulation).
Activators are needed for RNA polymerase to initiate transcription.
Inducers increase gene expression (by activating an activator or inactivating a repressor).
Co-repressors decrease gene expression (by activating a repressor or inactivating an activator).
Repressible systems regulate biosynthetic pathways based on end-product availability.
Constitutive mutants always transcribe operon genes, regardless of inducer presence.

Transcriptional Regulation Details

Regulators can be repressors, activators, or both.
Regulation Mechanisms: negative, positive, or both.
Activators: bind and recruit RNA polymerase; stabilize RNA polymerase binding to the promoter; promote open complex formation; promote frequency of initiation; can also inhibit repressors.
Repressors: simplest mechanism is steric hindrance (preventing RNA polymerase binding or proceeding); can change DNA structure, inhibiting RNA polymerase binding or proceeding; can act as anti-activators, affecting activator positive activity.

Operon Regulation

Catabolic operons degrade compounds for building molecules.
Biosynthetic operons synthesize molecules like amino acids, nucleotides, and vitamins.
Negative regulation: genes are usually on (constitutive) unless a repressor turns them off. Co-repressors bind to repressors, enabling binding to the operator, thus reducing expression.
A repressor needing a co-repressor to be active is called an aporepressor.

Positive Regulation (E. coli ara Operon)

Operon expression depends on an activator protein presence.
The ara operon encodes enzymes for arabinose utilization.
AraC is a divergently encoded transcriptional activator protein with dual function.
In the presence of arabinose, AraC changes conformation (P2 binding), activates the operon by binding to AraO2 and other operator sequences.
The absence of arabinose causes AraC to bind to different DNA sequences (P1) preventing transcription.
AraC has autoregulation, high concentrations prevent transcription of the operon from P1.

DNA-Binding Protein Motifs

Helix-turn-helix (HTH) motif is a common DNA-binding motif in proteins.
These proteins typically consist of two alpha-helices and a linker connecting them.
Amino acids in the recognition helix (helix 2) can interact and form hydrogen bonds with DNA bases in the major groove.

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Description

This quiz covers topics related to gene expression regulation in advanced microbiology. It explores various levels of regulation, including transcriptional mechanisms, roles of activators and repressors, and the effects of inducers and co-repressors. Test your understanding of how gene expression is fine-tuned in different cellular environments.