Regulation of Gene Expression in Bacteria

Overview of Prokaryotic Gene Regulation

Bacteria, such as E. coli, regulate gene expression to adapt to environmental changes and metabolic needs. Efficient genetic mechanisms control the transcription of specific genes, allowing bacteria to respond rapidly to environmental signals.

• Inducible enzymes: Produced only when specific substrates are present.

• Constitutive enzymes: Continuously produced regardless of environmental conditions.

• Repressible systems: Gene expression is inhibited by the presence of a specific molecule, often the end product.

• Positive and negative control: Negative control involves gene expression being shut off by a regulator molecule, while positive control requires a regulator molecule to stimulate transcription.

Lactose Metabolism and the lac Operon

Lactose Metabolism in E. coli

Lactose is a disaccharide composed of galactose and glucose. In E. coli, the presence of lactose induces the expression of enzymes required for its metabolism, making these enzymes inducible.

• Lactose acts as an inducer by stimulating transcription of the lac operon.

• Regulatory regions: Control transcription and are usually located upstream of gene clusters.

• Cis-acting elements: DNA sequences that regulate genes on the same molecule.

• Trans-acting elements: Molecules (often proteins) that bind to cis-acting sites to regulate gene expression.

lac Operon Structural Genes

The lac operon consists of three structural genes: lacZ, lacY, and lacA. These genes are transcribed as a single polycistronic mRNA, which is then translated into three distinct proteins.

• lacZ: Encodes β-galactosidase, which converts lactose into glucose and galactose.

• lacY: Encodes permease, facilitating lactose entry into the cell.

• lacA: Encodes transacetylase, involved in removing toxic by-products.

Regulation of the lac Operon: Negative Control

The lac operon is regulated by the lacI repressor gene, which produces an allosteric repressor protein. This protein binds to the operator region, inhibiting RNA polymerase and repressing transcription when lactose is absent.

• Negative control: Transcription occurs only when the repressor fails to bind the operator.

• Inducer (lactose): Binds to the repressor, causing a conformational change and preventing it from binding the operator, thus allowing transcription.

Constitutive Mutations in the lac Operon

Mutations in the lacI gene or operator region can lead to constitutive expression of the structural genes, meaning they are always transcribed regardless of lactose presence.

• lacI- mutation: Repressor protein is altered or absent and cannot bind the operator.

• OC mutation: Operator DNA sequence is altered and will not bind with a normal repressor molecule.

Super-Repressed Mutations

Mutations in the repressor gene can result in a repressor that always binds the operator, blocking transcription even in the presence of lactose.

Genetic Proof of the Operon Model: Merozygotes

Merozygotes are cells diploid for certain added genes. Analysis of lac expression in partial diploid cells supports the operon model, demonstrating the roles of the I gene (trans-acting) and O region (cis-acting).

Positive Control: Catabolite-Activating Protein (CAP) and the lac Operon

CAP and Catabolite Repression

CAP (catabolite-activating protein) exerts positive control over the lac operon. When glucose is present, CAP activity is diminished, reducing lac operon expression (catabolite repression).

• CAP binds to the CAP-binding site only when complexed with cyclic AMP (cAMP).

• Glucose inhibits adenylyl cyclase, lowering cAMP levels and preventing CAP binding.

• Maximum transcription: Requires both the repressor to be inactivated by lactose and CAP-cAMP to be bound to the promoter.

lac Operon Paradox

Permease is needed for lactose entry, but its expression depends on lactose induction. Occasional spontaneous dissociation of the repressor allows basal expression of permease, enabling lactose uptake.

Crystal Structure Analysis of Repressor Complexes

Lac Operon Regulatory Regions

Detailed structural analysis reveals three operator sites for repressor binding. In vivo, all three must be bound for maximum repression, forming a repression loop that prevents RNA polymerase access to the promoter.

The Tryptophan (trp) Operon: A Repressible Gene System

trp Operon Structure and Function

The trp operon in E. coli is a repressible system controlling the biosynthesis of tryptophan. It consists of five structural genes (trpE, trpD, trpC, trpB, trpA) transcribed as a polycistronic mRNA.

• trpP: Promoter, binding site for RNA polymerase.

• trpO: Operator, binds the repressor.

• Repressor: Binds to the operator in the presence of tryptophan, blocking transcription.

RNA-Mediated Regulation: Attenuation and Riboswitches

Attenuation Mechanism

Attenuation is a regulatory mechanism involving the secondary structure of mRNA. The leader sequence of the trp operon contains an attenuator, which forms different stem-loop structures depending on tryptophan availability.

• Antiterminator hairpin: Forms in the absence of tryptophan; ribosome stalls at tryptophan codons.

• Terminator hairpin: Forms in the presence of tryptophan; ribosome proceeds, terminating transcription.

Riboswitches

Riboswitches are alternative mRNA secondary structures that bind small ligands, causing conformational changes and regulating gene expression. They consist of two domains:

• Aptamer: Binds to the ligand.

• Expression platform: Capable of forming terminator structures.

Small Noncoding RNAs

Bacterial small noncoding RNAs regulate gene expression by binding to mRNAs. They can act as negative regulators by masking the ribosome-binding site or as positive regulators by preventing secondary structures that would otherwise inhibit translation.

Summary Table: Comparison of lac and trp Operon Regulation

Additional info: The attenuation mechanism is common to several amino acid biosynthetic operons, including threonine, histidine, leucine, and phenylalanine.