regulatory mechanisms

Common Regulatory Mechanisms in Bacteria

  • Gene Expression Regulation

    • Involves processes such as transcription and translation.

    • Also includes post-translational modification to alter the activity of enzymes and proteins.

    • All living organisms can be grouped into three domains, which differ in genome structure and the regulatory mechanisms utilized.

Regulation of Transcription Initiation

  • Basics of Transcription Regulation

    • Replacement of degraded enzymes.

    • Two types of genes related to transcription regulation:

    • Constitutive Genes

      • Genes that are continuously expressed by the cell.

    • Inducible Genes

      • Genes that code for enzymes only needed in specific environmental conditions (e.g., b-Galactosidase).

Inducible Genes

  • β-Galactosidase

    • An enzyme that becomes activated in the presence of lactose.

    • Catalyzes the hydrolysis of lactose into glucose and galactose.

    • Inducible enzymes are present only when their substrate, known as the inducer or effector molecule, is available.

    • Inducers: Allolactose (a form of lactose that activates b-galactosidase).

Repressible Genes

  • Characteristics

    • Enzymes functioning in biosynthetic pathways, whose production can be inhibited by the presence of their end products.

    • Example: An enzyme involved in amino acid synthesis will be repressed when the corresponding amino acid is available. This is true for Acetyl-CoA as a repressible gene's end product.

Control of Transcription Initiation by Regulatory Proteins

  • Regulatory Proteins and Their Roles

    • Regulatory proteins are responsible for the induction and repression of gene expression, exhibiting DNA-binding domains.

    • These proteins either inhibit transcription (negative control) or promote transcription (positive control).

    • Common structural motif: Helix-Turn-Helix

    • Consists of 20 amino acids, including two alpha helices and one beta turn, facilitating binding to DNA.

Negative Transcriptional Control

  • Mechanism

    • Involves the binding of regulatory protein at the DNA regulatory site, inhibiting transcription initiation.

  • Repressor Proteins:

    • Can exist in both active and inactive forms.

    • The presence of inducers or corepressors alters the activity of the repressor, thus reducing mRNA expression.

Positive Transcriptional Control

  • Mechanism

    • Involves binding of a regulatory protein at a regulatory region on DNA to promote transcription initiation.

  • Activation:

    • An inactive protein becomes activated.

    • An active protein can also become inactivated by inhibitors.

Examples of Transcriptional Control

  • Inducers and Corepressors

    • Inducers can activate inactivated proteins, while inhibitors can deactivate active proteins.

“Decision” Process in Gene Expression

  • Nature of Gene Regulation

    • Genes do not simply switch off; instead, the level of mRNA synthesis may decrease.

    • Enzymes of the catabolite pathway are synthesized only when their substrate is available, allowing for an efficient use of energy and materials.

Operon Structure

  • Definition

    • An operon consists of a promoter, operator, activator-binding sites, and structural genes.

    • Notable historical significance: discovered by Jacques Monod and François Jacob, recipients of the Nobel Prize in 1965.

    • Example: lac operon in E. coli, a well-studied negative control system.

Negative Control of the Lactose (lac) Operon

  • Structure

    • Comprises three structural genes coding for lactose uptake and metabolism.

    • The lac repressor (lacI) binds to the operator to inhibit transcription, ensuring enzymes are not produced unless lactose is present (inducible genes).

lac Repressor (LacI)

  • Structure and Function

    • Forms tetramers, each containing a helix-turn-helix DNA binding domain, binding at three operator sites (O1, O2, O3).

    • When lactose levels are low, the repressor binds to the operator and inhibits RNA polymerase from accessing the promoter.

    • When allolactose binds to the repressor, it no longer binds to the operator, allowing transcription to proceed.

Regulation of lac Operon

  • Functional Mechanism

    • Regulated by the catabolite activator protein (CAP) in response to glucose availability.

    • Allows preferential use of glucose over lactose for energy utilization.

lac Operon Transcriptional Regulation Scenarios

  1. Lactose but no glucose:

    • Transcription occurs; binding of RNA polymerase to promoter is enhanced by CAP.

  2. Lactose and glucose:

    • Transcription is inhibited by the presence of the repressor.

  3. Neither lactose nor glucose:

    • Repression occurs.

  4. Glucose but no lactose:

    • Transcription is inhibited by lack of CAP and presence of the repressor.

The Tryptophan (trp) Operon

  • Functionality

    • Has five structural genes coding for enzymes necessary for synthesizing tryptophan.

    • Regulated by a trp repressor, requiring negative transcriptional control that depends on tryptophan availability.

Regulation of Transcription Elongation

  • Overview

    • In addition to initiation regulation, elongation can also be controlled, especially in the trp operon.

    • Attenuation: A mechanism that leads to transcription termination within the leader region of the mRNA depending on the tryptophan levels in the cell.

  • Riboswitches: Recent discoveries indicate their role in regulating transcription termination.

Riboswitches (Sensory RNAs)

  • Description

    • Specialized RNAs whose folding patterns determine whether transcription will continue or terminate based on the binding of effector molecules.

    • Function primarily in transcriptional attenuation within gram-positive bacteria.

Comparison: DNA vs. mRNA

  • Components

    • Promoter region includes the RNA polymerase recognition site, with specific sequences:

    • Pribnow box, -35 and -10 region for initiation.

    • mRNA has a Shine-Dalgarno sequence and a start codon (AUG).

    • Template strands are distinguished in the context of transcription.

Regulation of Translation

  • Mechanism in Gram-negative Bacteria

    • mRNA translation regulated by small RNAs and effector binding elements at the 5’ end.

    • Translation initiation can be altered by the binding of these small RNA molecules, blocking the sequence, thereby preventing ribosomal translation.

Chemotaxis in E. coli

  • Functionality

    • Methyl-accepting chemotaxis proteins (MCPs) act as chemoreceptors in the membrane to bind environmental chemicals.

    • This initiates a series of interactions with cytoplasmic proteins that affect flagellar rotation:

    • Run: Counter-clockwise (CCW) movement of peritrichous flagella.

    • Tumble: Clockwise (CW) movement of peritrichous flagella.

Chemotaxis Response in E. coli

  • Two-component Regulatory System

    • Binding of chemoreceptors activates sensor kinase CheA, which autophosphorylates.

    • CheA then phosphorylates response regulator CheY, which governs the rotation of flagella based on its concentration within the cytoplasm.

  • Covalent Modification: Modifies chemotaxis receptors leading to different flagellar movements based on the state of CheA.

Quorum Sensing

  • Definition and Importance

    • A cell-to-cell communication method mediated by signaling molecules (e.g., N-acyl-homoserine lactone - AHL).

    • This system couples cell density with intercellular communication, impacting transcription regulation, gene activation for virulence, biofilm production, and morphological differentiation.

Quorum Sensing in V. fischeri

  • Mechanism of Action

    • High concentrations of AHL produced at elevated cell densities diffuse back into the cells, binding to transcriptional regulators, leading to the activation of transcription for genes required for AHL synthase (luxl) and bioluminescence.

Sporulation in Bacillus subtilis

  • Global Regulatory System

    • Involves phosphorelay and transcription initiation regulatory proteins, playing roles in conditions such as starvation and post-translational modification, inducing the production of alternative sigma factors.

Bacteria Response to Viral Infection

  • Defense Mechanisms

    • Microbes can methylate their DNA to differentiate between their own (methylated) and viral DNA (unmethylated).

    • Restriction Endonucleases: Enzymes that destroy viral DNA, functioning similarly to innate immune systems in Archaea and Bacteria, targeted through genetic engineering.

    • Viral DNA can modify itself to escape recognition and potentially increase infectivity.