Prokaryotic Gene Regulation

Chapter 11: Regulation of Gene Expression in Prokaryotes

Overview of Regulation

Transcriptional Regulation

  • Key components include regulatory sequences, activator and repressor proteins.

  • Regulatory sequences influence the binding and function of activators and repressors.

  • Positive Regulation: Activators enhance gene expression.

  • Negative Regulation: Repressors inhibit gene expression.

Allosteric Effectors

  • Molecules that modulate the activity of regulatory proteins (required for activation or inhibition).

Understanding Attenuation

  • Attenuation: A mechanism where the mRNA production of a gene is reduced after transcription has begun.

  • Key to bacterial operons, which are polycistronic mRNA sequences transcribed from a single promoter.

The Lac Operon

Lactose Control

  • Definition: The lac operon controls lactose metabolism in bacteria.

Activation of Transcription

  • Without Lactose: The repressor binds to the operator, preventing transcription.

  • With Lactose: Lactose acts as an inducer, binding to the repressor, allowing transcription.

Jacob and Monod Experiments

  • P+ O+ Z+ Y+ A+ - Wild-type operon configuration.

  • Studies focused on how mutations in repressor and operator influence transcription.

Operator Mutation Influence

  • Explores whether operator mutations affect gene expression in a cis manner (localized effect).

  • Mutations in the operator can affect the binding of the repressor, impacting transcription levels.

Non-functional Mutation in the I Gene

  • Examines how mutations influence transcription of lactose metabolism genes.

  • I- plasmid configurations can lead to either constitutive expression or repression based on context.

Gain-of-Function Mutation in the I Gene

  • Super repressor (Is): Mutant form of the repressor that binds to the operator, even in presence of lactose, inhibiting transcription.

Glucose Control of the Lac Operon

Glucose and cAMP Regulation

  • High glucose decreases cAMP levels, leading to reduced activation of the lac operon.

  • Low glucose increases cAMP, which enhances the binding of CAP (catabolite activator protein) to promote transcription.

Review of Lac Operon Regulation

  • Positive and negative regulatory mechanisms interact to control lactose metabolism gene expressions based on glucose levels.

Transcription States

  • (a) Glucose present, no lactose = no lac mRNA.

  • (b) Glucose present, lactose present = very little lac mRNA.

  • (c) No glucose, lactose present = abundant lac mRNA.

The Trp Operon

Attenuation Mechanism

  • Describes regulation of transcription in response to tryptophan levels.

Leader Sequence Function (trp L)

  • Leader sequence formation can stall ribosome during translation at high tryptophan levels, leading to termination of transcription.

Effect of UAG Codon Mutation

  • A mutation resulting in a UAG codon could prevent the trp operon from responding effectively to tryptophan levels, influencing overall regulation.