Regulation of Gene Expression (Campbell Biology in Focus, Chapter 15)

Regulation of Gene Expression

Introduction

Gene expression is the process by which information from a gene is used to synthesize functional gene products, such as proteins. Both prokaryotic and eukaryotic organisms regulate gene expression in response to environmental and developmental cues. Understanding these regulatory mechanisms is fundamental to the study of biology, as they underlie cellular function, differentiation, and adaptation.

Objectives of Chapter 15

• Identify the point at which control of gene expression usually occurs.

• Describe the usual action of regulatory proteins.

• Explain control of gene expression in the trp operon.

• Explain control of gene expression in the lac operon.

• Describe differential eukaryotic gene expression at the levels of chromatin structure, transcription, RNA processing, mRNA degradation, and protein processing.

Overview: The Importance of Gene Expression Regulation

• Both prokaryotes and eukaryotes alter gene expression in response to environmental conditions.

• Regulation of gene expression is crucial for cell specialization in multicellular organisms.

• Abnormalities in gene expression can lead to diseases, including cancer.

Gene Expression: From DNA to Protein

Central Dogma of Molecular Biology

• Transcription: The process by which a DNA sequence is copied into messenger RNA (mRNA).

• Translation: The process by which the mRNA sequence is used to synthesize a polypeptide (protein).

• Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid.

Example: The sequence of codons in mRNA determines the sequence of amino acids in a protein.

Gene Regulation in Prokaryotes

Operon Model

In bacteria, gene expression is often regulated at the level of transcription through the operon model. An operon is a cluster of functionally related genes controlled by a single promoter and operator.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Operator: DNA segment that acts as a regulatory "switch"; usually located within or near the promoter.

• Regulatory gene: Encodes a repressor protein that can bind to the operator to block transcription.

Types of Operons

• Repressible operon: Usually on; can be turned off by a repressor (e.g., trp operon).

• Inducible operon: Usually off; can be turned on by an inducer (e.g., lac operon).

The trp Operon (Repressible Operon)

• Controls genes for tryptophan synthesis in E. coli.

• Default state: ON (genes transcribed).

• When tryptophan is abundant, it acts as a corepressor by binding to the trp repressor protein, activating it.

• The active repressor binds the operator, blocking transcription.

The lac Operon (Inducible Operon)

• Controls genes for lactose metabolism in E. coli.

• Default state: OFF (repressor is active and bound to operator).

• When lactose is present, allolactose (an isomer of lactose) acts as an inducer by binding to the repressor, inactivating it.

• Inactive repressor detaches from the operator, allowing transcription.

Comparison of Repressible and Inducible Operons

Feature	Repressible Operon (trp)	Inducible Operon (lac)
Default State	ON	OFF
Regulation	Turned OFF by corepressor	Turned ON by inducer
Pathway Type	Anabolic (biosynthetic)	Catabolic (degradative)
Example	Tryptophan synthesis	Lactose metabolism

Positive Gene Regulation

• Some operons are also regulated by activator proteins.

• In E. coli, the cAMP receptor protein (CRP) is an activator that increases transcription of the lac operon when glucose is scarce.

• cAMP binds to CRP, enabling it to bind near the promoter and enhance RNA polymerase binding.

• When glucose is abundant, cAMP levels drop, CRP detaches, and transcription slows.

Gene Regulation in Eukaryotes

Levels of Gene Regulation

• Gene expression in eukaryotes is regulated at multiple stages:

  • Chromatin structure

  • Transcription

  • RNA processing (splicing, capping, polyadenylation)

  • mRNA degradation

  • Translation

  • Protein processing and degradation

Chromatin Structure and Epigenetic Regulation

• DNA is packaged with histone proteins into chromatin.

• Genes in tightly packed heterochromatin are usually not expressed.

• Chemical modifications to histones and DNA affect chromatin structure and gene expression.

Histone Modifications

• Histone acetylation: Addition of acetyl groups to histone tails loosens chromatin, promoting transcription.

• Histone methylation: Addition of methyl groups condenses chromatin, reducing transcription.

DNA Methylation

• Addition of methyl groups to cytosine bases in DNA.

• Heavily methylated genes are usually not expressed.

• Methylation patterns can be inherited during cell division (epigenetic inheritance).

Transcriptional Regulation

• Transcription factors bind to control elements (enhancers, promoters) to regulate gene expression.

• Combinatorial control: The specific combination of control elements and transcription factors determines gene expression.

• Coordinately controlled genes may be scattered across chromosomes but share common control elements.

Post-Transcriptional Regulation

• Alternative RNA splicing: Different mRNAs can be produced from the same primary transcript by including or excluding certain exons.

• This increases the diversity of proteins encoded by a genome.

Translational and Post-Translational Regulation

• Translation initiation can be regulated by proteins binding to mRNA.

• The lifespan of mRNA in the cytoplasm affects how much protein is produced.

• Protein processing (cleavage, chemical modification) and selective degradation (e.g., ubiquitin tagging) further regulate protein levels.

Noncoding RNAs in Gene Regulation

• Only about 1.5% of the human genome codes for proteins.

• Many noncoding RNAs (ncRNAs), such as ribosomal RNA (rRNA), transfer RNA (tRNA), and regulatory RNAs, play crucial roles in gene regulation.

• Regulatory ncRNAs can control gene expression at multiple levels, including chromatin remodeling, transcription, and mRNA stability.

Additional info: Examples of regulatory ncRNAs include microRNAs (miRNAs) and small interfering RNAs (siRNAs), which can bind to mRNA and block translation or promote degradation.