Prokaryotic Transcription Regulation: Operons and Gene Control in Bacteria

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Gene Expression and Function

Overview of Genetic Information Flow

Gene expression in all organisms follows a central dogma: DNA is transcribed into RNA, which is then translated into protein. Regulation at each step ensures the correct products are made at the right time and place.

DNA: Hereditary information; subject to replication and recombination.
RNA: Transcribed from DNA; serves as a template for protein synthesis.
Protein: Functional molecules produced via translation.
Gene Regulation: Controls product type, quantity, timing, and location.

Gene Regulation

Gene Regulation in Eukaryotes vs. Prokaryotes

Eukaryotes:
- Multiple levels of regulation (transcriptional, post-transcriptional, translational, post-translational).
- Transcriptional regulation is long-term but slow; mRNAs are stable and require time for degradation and production.
- Other regulatory forms allow rapid responses to diverse signals.
- Default gene state: Inhibited; regulators are mostly activators.
Prokaryotes:
- Limited regulatory need for cell type, tissue, and developmental stage differentiation.
- Regulation focuses on transcription; mRNAs are unstable and immediately available for translation.
- Default gene state: Activated; regulators are mostly repressors.

cis-acting Elements and trans-acting Factors

Definitions and Interactions

cis-acting elements: DNA sequences that regulate the gene they are part of (e.g., promoters, operators).
trans-acting factors: Proteins or RNAs produced from different genes that bind to cis-elements to regulate gene expression (e.g., repressors, activators).
Promoter: A type of cis-element; sigma/transcription factors are trans-factors.

Interaction between cis-elements and trans-factors is essential for transcriptional regulation.

Bacterial Transcription Control: Operons

Operon Structure and Function

An operon is a transcription unit containing several genes under the control of a shared promoter and terminator. It produces a polycistronic mRNA, allowing coordinated regulation of functionally related genes.

Each gene has separate start and stop codons for translation.
Operons enable efficient regulation of metabolic pathways.

Key cis-elements and trans-factors in Bacteria

cis-elements	trans-factors	Prevalence
Promoters	RNA polymerase (sigma factor)	in all genes
Operators	repressors	only in negatively controlled genes
Activator binding sites	activators	only in positively controlled genes

Additional info: Operators and activator binding sites are typically upstream of promoters, similar to eukaryotic regulatory elements.

Terminology in Bacterial Gene Regulation

Core Concepts

Active trans-factor: Able to bind its cis-element.
Inactive trans-factor: Unable to bind its cis-element.
Upstream regulation: Small molecules (inducers/corepressors) activate/inactivate trans-factors by binding and inducing conformational changes.
Downstream regulation: Trans-factors regulate target gene expression by binding to cis-elements.

Gene Expression Levels and Small Molecules

Induction: Upregulation; inducer binds to trans-factor.
Repression: Downregulation; corepressor binds to trans-factor.
Negative control: Operator + repressor.
Positive control: Activator binding site (ABS) + activator.

Lac Operon

Function and Products

The lac operon enables bacteria to metabolize β-galactoside sugars, such as lactose. It encodes:

β-galactosidase (lacZ): Breaks down lactose into glucose and galactose.
Permease (lacY): Transports lactose into the cell.
Transacetylase (lacA): Not essential for lactose metabolism.

Dual Control of Initiation

Positive inducible: Requires absence of glucose for activation.
Negative inducible: Requires presence of β-galactoside (lactose) for activation.
Only when glucose is absent and lactose is present is the lac operon induced.

Negative Inducible Regulation

Without lactose, expression is at a low basal level.
Addition of lactose (inducer) increases expression rapidly (~1000-fold).
When lactose is depleted, mRNA levels drop quickly, but protein levels persist longer.

Mechanism: Repressor/Operator Interaction

Repressor protein has three domains: DNA binding, core (dimerization/inducer binding), and tetramerization.
Repressors can form dimers or tetramers; tetramer binding to multiple operators increases repression.
Operator sequence is a palindrome; repressor binds via major grooves.

Mechanism: Repressor/Inducer Interaction

Inducers bind to the core domain, changing repressor conformation and reducing DNA binding ability (allosteric control).
Inducer binding separates DNA binding domains, preventing simultaneous binding to operator.

Lac Operon Inducers

Inducers are highly specific; usually substrates or products of regulated enzymes.
Natural inducer: allolactose (a by-product of β-galactosidase activity).
Artificial inducer: IPTG.

Mutations in the Lac Operon

Mutations in operator or repressor genes can abolish interactions, leading to constitutive expression or repression.
Constitutive expression: Gene is always on.
Constitutive repression: Gene is always off.

Mutation in	Molecular consequence	Operon expression
Operator	Abolish repressor binding	Constitutive expression
Repressor	Abolish DNA binding	Constitutive expression
Repressor	Abolish binding or response to inducer	Constitutive repression

Dual Control and Activator Requirement

Some operons (e.g., lac) have weak promoters and require activators for efficient polymerase recruitment.
Lac operon is also positively inducible, responding to glucose availability.

Lac Operon Activator: cAMP Receptor Protein (CRP/CAP)

Activator binding site binds CRP (CAP), a trans-factor.
CRP is active when bound to cAMP (produced when glucose is low).
CRP binding promotes RNA polymerase recruitment and transcription.

CRP states: Active (binds DNA), Inactive (does not bind DNA).

Regulation by Glucose and cAMP

Glucose inhibits adenylate cyclase, reducing cAMP production.
Low glucose → high cAMP → active CRP → lac operon expression.
High glucose → low cAMP → inactive CRP → no lac operon expression.

Summary Table: Lac Operon Responses

Glucose	cAMP	CRP binds	Lactose	Repressor binds	Level of transcription
+	-	-	-	+	Very low
+	-	-	+	-	Low
-	+	+	-	+	Very low
-	+	+	+	-	High

Trp Operon

Function and Regulation

The trp operon encodes genes for tryptophan synthesis. It is regulated by its own product, tryptophan, which acts as a corepressor.

Negative feedback prevents excessive tryptophan synthesis.
Trp promoter is negative repressible: Tryptophan activates the trp repressor.

Trp Operon Structure

Three transcription units, each with a trp operator.
Encodes enzymes for tryptophan synthesis and the repressor protein.

Trp Operon Regulation Summary

Low tryptophan: Repressor inactive, synthesis enzymes produced.
High tryptophan: Repressor activated, operon repressed, synthesis reduced.

Dual Control: Initiation and Termination

Initiation: Operator/repressor responds to free tryptophan.
Termination: Attenuation responds to tRNA-Trp levels.
Parallel regulation: Two tiers of expression levels.

Trp Operon Attenuation

Attenuation is a secondary control mechanism that responds to tRNA-Trp levels.
Attenuation alone cannot fully repress operon expression but can reduce it by 10-fold.
Combined with repressor control, expression can be reduced from 700x to 70x.

Free trp	tRNA-Trp	Operator repression	Attenuation repression	Overall expression
high	high	in action	in action	Very low, 1x (basal level)
high	low	in action	relieved	Medium, 70x
low	relieved	no action	relieved	High, 700x

Attenuation Mechanism

Leader sequence contains trp codons; ribosome stalling at these codons (due to low tRNA-Trp) prevents formation of terminator hairpin, allowing transcription to continue.
High tRNA-Trp allows formation of terminator hairpin, causing premature transcription termination.

Key Terms and Concepts

Operon: Cluster of genes under control of a single promoter and operator.
Polycistronic mRNA: mRNA encoding multiple proteins.
Inducer: Molecule that inactivates a repressor, allowing gene expression.
Corepressor: Molecule that activates a repressor, inhibiting gene expression.
Attenuation: Regulatory mechanism controlling transcription termination based on metabolite levels.
CRP/CAP: cAMP receptor protein/catabolite activator protein; activates transcription in response to low glucose.