Gene Regulation in Bacteria

Introduction to Gene Regulation

Gene regulation refers to the control of the level and timing of gene expression. In bacteria, gene regulation ensures that proteins are produced only when needed, optimizing resource use and cellular function. Genes that are always expressed at constant levels are called constitutive genes. Regulation is crucial for metabolism, response to environmental stress, and cell division, and can occur at multiple stages of gene expression.

• Constitutive genes: Genes expressed continuously at constant levels.

• Regulated genes: Genes whose expression varies depending on cellular needs.

• Key roles of regulation: Metabolism, stress response, cell division.

Points of Regulation in Bacterial Gene Expression

Gene expression in bacteria can be regulated at several stages:

• Transcriptional regulation: Control of RNA synthesis by regulatory proteins or mechanisms like attenuation.

• Translational regulation: Control of protein synthesis from mRNA by repressor proteins, riboswitches, or antisense RNA.

• Posttranslational regulation: Control of protein activity via feedback inhibition or covalent modification.

Summary Table: Points of Regulation

Transcriptional Regulation

Regulatory Transcription Factors

Transcriptional regulation is primarily mediated by proteins called regulatory transcription factors (RTFs):

• Repressors: Bind DNA and inhibit transcription (negative control).

• Activators: Bind DNA and increase transcription (positive control).

Small Effector Molecules

Small molecules can modulate the activity of RTFs:

• Inducers: Increase transcription by inactivating repressors or activating activators.

• Inhibitors: Bind activators and prevent DNA binding.

• Corepressors: Bind repressors and enable DNA binding.

Summary Table: Effector Molecules and Their Effects

Regulation of the lac Operon

Structure and Function

The lac operon in E. coli is a classic example of gene regulation, controlling the metabolism of lactose. It consists of:

• Structural genes: lacZ (β-galactosidase), lacY (lactose permease), lacA (galactoside transacetylase).

• Regulatory elements: Promoter (binds RNA polymerase), Operator (binds repressor), CAP site (binds activator), Terminator.

• lacI gene: Encodes the lac repressor, expressed constitutively from its own promoter.

Negative Control: Repressor Mechanism

In the absence of lactose, the lac repressor binds the operator, blocking transcription. When lactose is present, it is converted to allolactose, which binds the repressor, causing it to release from the operator and allowing transcription.

• Allosteric regulation: Allolactose binding changes repressor conformation.

• Inducible system: The operon is induced (turned on) by the presence of lactose.

Positive Control: Catabolite Repression and CAP

When glucose is scarce, cyclic AMP (cAMP) levels rise. cAMP binds to the Catabolite Activator Protein (CAP), which then binds the CAP site and enhances RNA polymerase binding, increasing transcription. When glucose is present, cAMP levels are low, CAP does not bind, and transcription is reduced.

• cAMP-CAP complex: Inducible, positive control of the operon.

• Catabolite repression: Glucose inhibits cAMP production, reducing operon expression.

Summary Table: lac Operon Regulation Under Different Conditions

Mutations Affecting the lac Operon

• lacI- (repressor null): No functional repressor is made; operon is constitutively expressed.

• lacIs (super-repressor): Repressor cannot be inactivated by allolactose; operon is always repressed.

• Oc (operator constitutive): Operator cannot bind repressor; operon is constitutively expressed (cis-acting).

Multiple Operator Sites and DNA Looping

The lac operon contains three operator sites (O1, O2, O3). O1 is the primary site; O2 and O3 are auxiliary. Binding of the repressor to two operators simultaneously causes DNA looping, enhancing repression. Loss of O1 abolishes repression, while loss of O2 or O3 reduces repression efficiency.

Regulation of the trp Operon

Structure and Function

The trp operon in E. coli encodes enzymes for tryptophan biosynthesis. It is a repressible system, typically ON but turned OFF when tryptophan is abundant.

• Structural genes: trpE, trpD, trpC, trpB, trpA.

• Regulatory genes: trpR (repressor protein), trpL (leader peptide, involved in attenuation).

• Regulatory elements: Promoter, operator, leader, attenuator.

Negative Control: Repression by Tryptophan

When tryptophan is low, the trp repressor is inactive and cannot bind the operator, so the operon is transcribed. When tryptophan is high, it acts as a corepressor, binding to the repressor and enabling it to bind the operator, blocking transcription.

Attenuation Mechanism

Attenuation is a second regulatory mechanism that relies on the coupling of transcription and translation in bacteria. The leader region (trpL) of the mRNA can form alternative stem-loop structures depending on tryptophan levels:

• High tryptophan: Ribosome quickly translates leader peptide, allowing formation of the 3-4 terminator stem-loop, causing premature termination of transcription.

• Low tryptophan: Ribosome stalls at Trp codons, allowing formation of the 2-3 anti-terminator stem-loop, so transcription continues into the structural genes.

Summary Table: trp Operon Regulation

Inducible vs. Repressible Operons

• Inducible operons: Usually involved in catabolism (e.g., lac operon); substrate acts as inducer.

• Repressible operons: Usually involved in anabolism (e.g., trp operon); product acts as corepressor.

Translational and Posttranslational Regulation

Translational Regulation

Translation can be regulated by proteins (translational repressors) or by antisense RNA:

• Translational repressors: Bind to mRNA near the Shine-Dalgarno sequence or elsewhere to block ribosome binding.

• Antisense RNA: Small RNA molecules (e.g., micF) base-pair with target mRNA (e.g., ompF), forming double-stranded RNA that blocks translation.

Posttranslational Regulation

• Feedback inhibition: The end product of a metabolic pathway inhibits an early enzyme, preventing overproduction.

• Covalent modification: Addition of chemical groups (phosphate, acetyl, methyl) to proteins can reversibly alter their activity.

Riboswitches

Definition and Mechanism

Riboswitches are regulatory segments of mRNA that can adopt different secondary structures in response to binding small molecules, thereby controlling gene expression at the level of transcription, translation, RNA stability, or splicing.

• Estimated to regulate 3-5% of bacterial genes.

• Switch between active and inhibitory conformations upon ligand binding.

Examples of Riboswitch Regulation

• Transcriptional regulation (e.g., TPP riboswitch in B. subtilis): Low TPP: antiterminator forms, transcription proceeds. High TPP: terminator forms, transcription stops.

• Translational regulation (e.g., TPP riboswitch in E. coli): Low TPP: Shine-Dalgarno sequence accessible, translation occurs. High TPP: Shine-Dalgarno sequence sequestered, translation blocked.

Summary Table: Riboswitch States

Key Terms and Concepts

• Operon: A cluster of genes under the control of a single promoter and regulatory elements, transcribed as a polycistronic mRNA.

• Attenuation: Regulation of transcription termination based on leader peptide translation and mRNA secondary structure.

• Allosteric regulation: Regulation of protein activity by binding of an effector molecule at a site other than the active site.

• Feedback inhibition: End product of a pathway inhibits an early enzyme in the pathway.

• Riboswitch: mRNA element that changes conformation upon ligand binding to regulate gene expression.

Formulas and Equations

• cAMP synthesis:

• Repression fold change (example from lac operon):

Examples and Applications

• lac operon: Inducible system for lactose metabolism; classic model for negative and positive transcriptional regulation.

• trp operon: Repressible system for tryptophan biosynthesis; model for repression and attenuation.

• Osmoregulation in E. coli: Antisense RNA (micF) regulates translation of ompF mRNA in response to osmolarity.

• TPP riboswitch: Regulates thiamin biosynthesis genes in response to TPP levels in both B. subtilis (transcription) and E. coli (translation).