Regulation of Gene Expression

Introduction

Regulation of gene expression is a fundamental process that allows cells to control which genes are active, ensuring efficiency and adaptability. Selective gene expression is crucial for cellular specialization and response to environmental changes. The earliest insights into gene regulation came from studies in bacteria.

Bacterial Gene Regulation

Constitutive vs. Regulated Genes

• Constitutive genes: Expressed continuously, regardless of environmental conditions.

• Regulated genes: Expression is controlled to meet cellular needs, often encoding enzymes required only under specific conditions.

Adaptive Enzyme Synthesis

• Cells regulate enzyme concentrations by controlling transcription in response to cellular needs, a process known as adaptive enzyme synthesis.

Induction and Repression in Metabolic Pathways

• Catabolic pathways (degradative): Enzyme synthesis is induced by the presence of substrate (e.g., lactose).

• Anabolic pathways (synthetic): Enzyme synthesis is repressed by the presence of the end product (e.g., tryptophan).

Substrate Induction and End-Product Repression

• Substrate induction: Enzymes are produced only when their substrate is present. Example: β-galactosidase is induced by lactose.

• End-product repression: Enzyme synthesis is inhibited when the end product accumulates. Example: Tryptophan represses its own biosynthetic enzymes.

Triggers for Induction and Repression

• Small organic molecules act as triggers: substrates for catabolic pathways, end products for anabolic pathways.

The Operon Model

Definition and Structure

• An operon is a cluster of genes with related functions, regulated together by a single promoter and operator.

• Operons are common in prokaryotes, rare in eukaryotes.

The lac Operon

• Consists of three structural genes: lacZ (β-galactosidase), lacY (galactoside permease), lacA (transacetylase).

• Regulated by the lacI gene, which encodes the lac repressor protein.

• Transcription produces a polycistronic mRNA coding for all three proteins.

Negative Regulation by the lac Repressor

• The lac repressor binds the operator, blocking RNA polymerase and preventing transcription.

• When lactose (specifically, allolactose) is present, it binds the repressor, causing a conformational change that releases the operator and allows transcription.

• The repressor is an allosteric protein with two conformational states.

Positive Regulation by Catabolite Activator Protein (CAP)

• Glucose inhibits lac operon expression via catabolite repression.

• When glucose is absent, cAMP levels rise, cAMP binds CAP, and the CAP-cAMP complex enhances RNA polymerase binding to the promoter.

Table: Regulation of the lac Operon

Mutational Analysis of the lac Operon

• Mutations in structural genes (lacZ, lacY) produce defective enzymes.

• Operator mutations (Oc) cause constitutive expression.

• Promoter mutations (Plac–) reduce transcription.

• Regulatory gene mutations (I–, Is) affect repressor function.

Table: Genetic Analysis of Mutations Affecting the lac Operon

Cis-Trans Test Using Partially Diploid Bacteria

• Cis-acting mutations: Affect only genes physically linked (e.g., operator mutations).

• Trans-acting mutations: Affect both copies of the operon (e.g., repressor protein mutations).

The trp Operon: Repressible Operon

• The trp operon encodes enzymes for tryptophan biosynthesis.

• Regulated by the trpR gene, which encodes the trp repressor.

• Tryptophan acts as a corepressor, activating the repressor to bind the operator and inhibit transcription.

Attenuation in the trp Operon

• Leader sequence in the trp operon mRNA can form hairpin loops that regulate transcription termination.

• When tryptophan is scarce, ribosome stalls, antiterminator hairpin forms, transcription continues.

• When tryptophan is abundant, terminator hairpin forms, transcription stops.

Table: Attenuation Mechanism in trp Operon

Riboswitches

• Regulatory RNA elements in mRNA leader regions that bind small molecules, altering mRNA structure to control transcription or translation.

• Example: Riboflavin operon in Bacillus subtilis regulated by FMN binding.

CRISPR/Cas System

• Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) and associated (Cas) proteins provide adaptive immunity against viruses in bacteria and archaea.

• CRISPR sequences incorporate viral DNA as spacers; Cas proteins process and target invading DNA for cleavage.

Eukaryotic Gene Regulation: Genomic Control

Cell Differentiation and Genomic Equivalence

• Multicellular eukaryotes consist of specialized cell types, each expressing a subset of genes.

• Most cells retain the complete genome (genomic equivalence), demonstrated by cloning experiments (carrots, frogs, sheep).

• Totipotency: The ability of a nucleus to direct development of an entire organism.

Stem Cells

• Pluripotent stem cells: Can form all cell types except placenta/support membranes.

• Multipotent stem cells: Can form several cell types within a limited range.

• Therapeutic cloning aims to produce patient-matched stem cells for regenerative medicine.

Levels of Eukaryotic Gene Regulation

• Gene expression is regulated at five main levels:

  1. Genome (DNA rearrangement, chromatin structure)

  2. Transcription

  3. RNA processing and export

  4. Translation

  5. Posttranslational events

DNA Rearrangements

• Transposons: Mobile genetic elements that can move within the genome.

• Lymphocyte receptor gene rearrangements: Antibody diversity is generated by recombination of V, D, J, and C segments.

• Somatic hypermutation: Further increases antibody diversity via error-prone repair.

Table: Antibody Gene Rearrangement

Chromatin Decondensation and Gene Expression

• Gene expression requires chromatin decondensation to expose promoter regions.

• Chromosome puffs: Visible regions of decondensed chromatin in polytene chromosomes, correlating with active transcription.

• DNase I sensitivity: Transcriptionally active chromatin is more susceptible to DNase I digestion, indicating uncoiled DNA.

Key Terms and Concepts

• Operon: Cluster of genes regulated together.

• Inducible operon: Turned on by substrate presence (e.g., lac operon).

• Repressible operon: Turned off by end product (e.g., trp operon).

• Allosteric protein: Protein whose activity is regulated by binding of an effector molecule.

• Polycistronic mRNA: mRNA encoding multiple proteins.

• Riboswitch: Regulatory RNA element controlling gene expression.

• CRISPR/Cas: Bacterial immune system against viruses.

• Genomic equivalence: All cells contain the same genome.

• Totipotency: Ability to form an entire organism from a single cell.

• Pluripotency: Ability to form most cell types.

• Transposon: Mobile DNA element.

• Chromatin decondensation: Unfolding of chromatin for gene expression.

Equations and Formulas

• Antibody diversity (heavy chain):

Additional info: Some figures referenced in the notes (e.g., Figures 20.1–20.16) are not included, but their content has been summarized in the text. Mutational analysis tables have been reconstructed based on context. The notes cover both prokaryotic and eukaryotic gene regulation, including operon models, mutational analysis, chromatin structure, and immune system gene rearrangement.