Prokaryotic transcription 2

Protein Expression Control

• Protein Synthesis Regulation: Regulation of protein synthesis involves the control of messenger RNA (mRNA) production.

  • More mRNA synthesis leads to increased protein production.

  • Reducing mRNA synthesis leads to decreased protein production.

Types of Genes in Bacteria

• Constitutive Genes:

  • Always turned on; continuously synthesized.

  • Example: Lac repressor, a housekeeping gene involved in essential metabolic processes like the TCA cycle.

• Regulated Genes:

  • Synthesis changes based on environmental factors, such as food availability.

  • Example: Transition from glucose to lactose as a food source.

  • Environmental changes can trigger the activation or deactivation of groups of genes necessary for survival.

Operon Model in Bacteria

• Operons:

  • Groups of adjacent genes that are co-regulated and transcribed as a single unit.

  • Genes in the same pathway are turned on simultaneously, resulting in polycistronic mRNA.

  • Example: The lac operon, involved in lactose metabolism in E. coli.

• Polycistronic RNA:

  • A single mRNA transcript that encodes multiple proteins, facilitating coordinated expression of related genes.

  • Contains multiple start and stop codons for each gene.

Lac Operon Specifics

• Components of the Lac Operon:

  • Genes: lacZ (beta-galactosidase), lacY (permease), lacA (transacetylase).

  • Regulation: Genes are only expressed when lactose is present and glucose is absent.

  • Functionality depends on the activity of RNA polymerase and the presence of regulatory molecules like allolactose.

• Regulation Mechanisms:

  • Negative Regulation: The lac repressor binds to the operator site to inhibit transcription when lactose is absent.

  • Positive Regulation: CAP (catabolite activator protein) binds when cyclic AMP (cAMP) levels are high (indicating low glucose), enhancing RNA polymerase affinity for the promoter.

Promoter and Transcription Factors

• Promoter Site:

  • Contains consensus sequences recognized by RNA polymerase.

  • Key regions include the -10 (TATA box) and -35 sites, which facilitate binding and transcription initiation.

• Transcription Regulation:

  • Binding of RNA polymerase is affected by mutations that alter affinity for the promoter.

  • Down mutations decrease transcription, while up mutations increase it.

RNA Polymerase Composition and Function

• RNA Polymerase Structure:

  • Composed of multiple subunits: 2 alpha, beta, beta prime, and an omega subunit.

  • The sigma factor is crucial for promoter recognition and initiation of transcription.

• Sigma Factors:

  • Different sigma factors (e.g., sigma70, sigma32) recognize distinct promoter sequences under various conditions (housekeeping vs. stress response).

Growth Phases and Gene Regulation

• Bacterial Growth Curve:

  • Lag Phase: Initial adjustment to the environment; gene expression prepares for growth.

  • Exponential Phase: Rapid growth, high RNA and protein synthesis.

  • Stationary Phase: Triggered by resource depletion, indicated by changes in transcription factors.

  • Death Phase: Results from continued resource depletion.

Termination of Transcription

• Mechanisms:

  • Intrinsic Termination: Involves a hairpin loop in mRNA followed by a run of U's, which reduces the stability of the RNA-DNA hybrid.

  • Rho-Dependent Termination: Involves a rho protein that unwinds the RNA-DNA complex after catching up to RNA polymerase when it pauses.

Summary of Key Concepts

• Initiation: Binding of RNA polymerase to the promoter, facilitated by sigma factors and consensus sequences.

• Elongation: RNA polymerase synthesizes mRNA from the DNA template.

• Termination: RNA polymerase halts transcription at specific sequences, releasing the new mRNA.

• Regulation Importance: Proper regulation of transcription is crucial for bacteria to adapt to environmental changes and optimize energy use, demonstrating the complexity of gene expression control.