Regulation of Gene Expression (Chapter 18): Study Notes

Regulation of Gene Expression

Introduction

The regulation of gene expression is a fundamental process that allows cells to control which genes are expressed, when, and to what extent. This regulation is essential for cellular function, adaptation, and development in both prokaryotes and eukaryotes.

Transcription Regulation in Bacteria

Overview

• Natural selection favors bacteria that express only the genes needed for survival.

• Gene expression can be regulated by feedback inhibition or gene regulation.

• In feedback inhibition, the end product of a metabolic pathway inhibits enzyme activity, shutting down further synthesis.

• Cells can also regulate enzyme production by controlling the expression of the genes encoding those enzymes.

Operon Model

• The operon is a cluster of functionally related genes controlled by a single "on-off switch" called the operator.

• The operator is a DNA segment within or near the promoter.

• An operon includes the operator, promoter, and the genes they control.

Regulation by Repressors and Corepressors

• The operon can be switched off by a repressor protein, which binds to the operator and blocks RNA polymerase.

• The repressor is encoded by a separate regulatory gene.

• A corepressor is a molecule that cooperates with a repressor to switch an operon off (e.g., tryptophan in E. coli).

The trp Operon (Repressible Operon)

• By default, the trp operon is on; genes for tryptophan synthesis are transcribed.

• When tryptophan is present, it acts as a corepressor, activating the repressor and turning the operon off.

• The trp operon is an example of a repressible operon (usually on, can be turned off).

The lac Operon (Inducible Operon)

• The lac operon contains genes for enzymes that hydrolyze and metabolize lactose.

• It is controlled by a main operator and promoter, and a regulatory gene (lacI) encodes a repressor protein.

• By default, the lac repressor is active and switches the operon off.

• An inducer (allolactose, an isomer of lactose) inactivates the repressor, turning the operon on.

• The lac operon is an example of an inducible operon (usually off, can be turned on).

Comparison of Repressible and Inducible Operons

Type	Default State	Regulation Mechanism	Example
Repressible	On	Turned off by repressor + corepressor	trp operon
Inducible	Off	Turned on by inducer inactivating repressor	lac operon

Catabolic vs. Anabolic Pathways

• Inducible enzymes (e.g., lac operon) function in catabolic pathways; synthesis is induced by a chemical signal.

• Repressible enzymes (e.g., trp operon) function in anabolic pathways; synthesis is repressed by high levels of the end product.

Positive Gene Regulation

• Some operons are also regulated by positive control via activator proteins (e.g., CRP/cAMP in the lac operon).

• When glucose is scarce, CRP binds cAMP, attaches to the promoter, and increases RNA polymerase affinity, enhancing transcription.

• When glucose is abundant, CRP detaches, and transcription returns to a low level.

Regulation of Gene Expression in Eukaryotes

Overview

• All organisms regulate gene expression in response to internal and external signals.

• In multicellular organisms, gene regulation is essential for cell specialization (differentiation).

Differential Gene Expression

• Nearly all cells in an organism contain the same genome.

• Differences in cell types arise from differential gene expression—the expression of different genes by cells with the same genome.

• Abnormal gene expression can lead to diseases, including cancer.

• Gene expression is regulated at many stages, often equated with transcription.

Regulation of Chromatin Structure

• Genes in tightly packed heterochromatin are usually not expressed.

• In euchromatin, gene transcription is influenced by nucleosome positioning and DNA attachment sites.

Histone Modifications and DNA Methylation

• Histone acetylation (addition of acetyl groups) opens chromatin structure, promoting transcription.

• Methylation of histone tails condenses chromatin, reducing transcription.

• Phosphorylation near methylated amino acids can have the opposite effect, loosening chromatin.

• DNA methylation (addition of methyl groups to DNA bases, usually cytosine) is associated with reduced transcription and long-term gene inactivation.

• In genomic imprinting, methylation regulates expression of maternal or paternal alleles at the start of development.

Epigenetic Inheritance

• Chromatin modifications can be inherited without altering the DNA sequence—this is called epigenetic inheritance.

• Epigenetic differences may explain why identical twins can have different disease susceptibilities.

Regulation of Transcription Initiation

• Chromatin-modifying enzymes control gene expression by altering DNA accessibility.

• General transcription factors and RNA polymerase II interact with the promoter for basal transcription.

• High levels of transcription require control elements (noncoding DNA segments) and specific transcription factors.

• Proximal control elements are near the promoter; distal control elements (enhancers) are farther away.

• Activators bind enhancers to stimulate transcription; repressors inhibit transcription.

• Protein-mediated DNA bending brings activators into contact with mediator proteins, which interact with general transcription factors to assemble the preinitiation complex.

Summary Table: Key Regulatory Elements in Eukaryotic Transcription

Element	Location	Function
Promoter	Upstream of gene	Binding site for RNA polymerase and general transcription factors
Proximal control elements	Near promoter	Bind specific transcription factors
Enhancers (distal control elements)	Far from promoter	Bind activators or repressors to regulate transcription
Mediator proteins	Between activators and transcription machinery	Facilitate assembly of preinitiation complex

Additional info: These notes cover the first half of Chapter 18, focusing on prokaryotic and eukaryotic gene regulation at the transcriptional level. Further sections would include post-transcriptional regulation, noncoding RNAs, and gene regulation in development and disease.