Molecular Regulation Chapter 10

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Questions and Answers

What is the primary reason cells do not express every gene at maximal levels?

Physical space limitations

Energy and resource conservation

Contradictory functions

All of the above (correct)

At what level does gene regulation primarily occur in bacteria?

Alteration of DNA sequence

Translational control

Post-translational control

Control of transcription (correct)

How do regulatory proteins typically bind to DNA?

Only as monomers

Either as monomers or dimers (correct)

In a linear fashion along the DNA strand

Only when bound to other proteins

What differentiates repressors from activators in gene regulation?

Repressors prevent gene expression, while activators stimulate it Signup and view all the answers

What role does the sensor kinase play in two-component signal transduction systems?

It binds environmental signals Signup and view all the answers

Which statement about transcription repressors is incorrect?

All repressors require ligands to function Signup and view all the answers

What do activators rely on to effectively bind to DNA sequences?

Binding of a ligand Signup and view all the answers

What describes the nature of regulatory proteins' interaction with DNA?

They often recognize and bind to symmetrical sequences Signup and view all the answers

What role does the response regulator play once stimulated by the activated sensor kinase?

It binds DNA to regulate gene expression. Signup and view all the answers

What happens to the lac operon in the absence of lactose?

LacI binds the operators, preventing RNA polymerase from initiating transcription. Signup and view all the answers

How does allolactose affect the function of LacI?

It reduces LacI's affinity for the operator. Signup and view all the answers

What is required for the maximum expression of the lactose operon?

Binding of the cAMP-CRP complex and removal of LacI. Signup and view all the answers

Which protein is responsible for transporting lactose into the cell?

Lactose Permease. Signup and view all the answers

What are the components of the lac operon?

lacZ, lacY, lacA, LacI. Signup and view all the answers

What effect does cAMP have during carbon starvation?

It accumulates and complexes with CRP. Signup and view all the answers

What is the function of β-Galactosidase in lactose metabolism?

It cleaves lactose into glucose and galactose. Signup and view all the answers

Study Notes

Chapter 10: Molecular Regulation

Overview:
- Introduction to gene regulation
- Transcription repressors and activators
- The lactose operon

Introduction

A bacterial genome encodes many different proteins
Cells do not express all genes at maximum levels, this is due to:
- Physical space limitations
- Energy and resource conservation
- Contradictory functions

Mechanisms of Gene Regulation

Microbes utilize various mechanisms to sense internal/external environments and synthesize specific proteins
Gene regulation occurs at multiple levels including:
- Alteration of DNA sequence
- Control of transcription
- Control of mRNA stability
- Translational control
- Post-translational control

Transcription Repressors and Activators

Transcription Initiation: Major control site in bacteria, often controlled by regulatory proteins.
Regulatory Proteins:
- Bind to DNA at or near gene promoters
- Stimulate or inhibit RNA polymerase binding to promoter
- Interact with DNA's major groove, often forming dimers
DNA Targets:
- Exhibit symmetry (inverted repeats)
- Regulatory proteins often bind as dimers (each monomer to one repeat sequence)
Mechanisms:
- Intracellular changes detected by regulatory proteins binding specific low-molecular-weight compounds (ligands)
- Genes encoding regulatory proteins are transcribed separately from target genes

Sensing the Extracellular Environment

Two-Component Signal Transduction Systems:
- Sensor Kinase: Membrane-bound enzyme, binds environmental signals and transfers a phosphate group from ATP to a target protein
  - Sensory domain interacts with the environment, kinase domain inside the cell
- Response Regulator: Cytosolic protein, activated by sensor kinases binding to DNA, affecting gene expression
  - Can either stimulate or repress gene expression
  - Down-regulated by phosphatases removing the phosphate group

The Lactose Operon

Historical Context:
- 1961: Monod and Jacob proposed gene regulation in E. coli
- 1965 Nobel Prize for gene regulation work
Lactose Metabolism:
- Disaccharide (glucose + galactose), carbon and energy source
Requires:
- Lactose permease (LacY): Transports lactose into the cell
- β-Galactosidase (LacZ): Breaks down lactose into monosaccharides or rearranges it to allolactose

Regulation of the lac Operon

Operon Structure:
- lacZ, lacY, lacA genes form an operon (regulated by a single promoter (PlacZYA) and operator sequences (lacO))
- Lactose repressor (LacI) is encoded by a regulatory gene upstream of the operon and constantly expressed

Lactose Presence

Mechanism:
- In absence of lactose, LacI binds operators (lacO and lacOI), preventing RNA polymerase from initiating transcription.
- In presence of lactose, allolactose binds LacI, decreasing its affinity for the operator, allowing RNA polymerase to initiate transcription of lacZYA genes.
cAMP and CRP
- Mechanism:
  - cAMP accumulates when cell is starving for carbon and combines with cAMP receptor protein (CRP)
- Maximum lactose operon expression requires removing LacI and binding of cAMP-CRP complex to DNA sequence upstream of transcription start site
  - cAMP-CRP complex activates RNA polymerase.
Mechanism:
- Catabolite Repression: Catabolism of one nutrient (e.g., glucose) represses operon expression in favour of a more favourable nutrient (eg glucose)
Diauxic Growth
- Biphasic growth curve of a culture growing on two carbon sources.
Inducer Exclusion
- Glucose transport (phosphotransferase system (PTS)) inhibits LacY permease
- Prevents lactose from entering the cell and inducing the lac operon
- In absence of glucose, phosphorylated forms of glucose-specific IIAGlc and IIBGlc accumulate, allowing LacY to transport lactose and induce lac operon.

Studying That Suits You

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Description

Explore the fundamental concepts of gene regulation in this quiz based on Chapter 10. This chapter covers transcription repressors, activators, and the lactose operon, detailing how microbes adapt their gene expression. Test your understanding of the mechanisms that control gene expression levels at various stages.