Regulation of Gene Expression

Overview

The regulation of gene expression is a fundamental process in biochemistry, controlling when, where, and how much of a gene product is produced. This chapter focuses on the molecular mechanisms that regulate gene expression in both prokaryotes and eukaryotes, with emphasis on transcriptional and translational control, epigenetic modifications, and RNA-based regulation.

Regulation of Transcription in Bacteria

The lac Operon: A Model System

The lac operon in Escherichia coli is a classic example of transcriptional regulation in prokaryotes. It enables the bacterium to utilize lactose as an energy source only when glucose is absent and lactose is present.

• Operon Structure: The lac operon consists of three structural genes (lacZ, lacY, lacA), a promoter, and an operator region.

• Polycistronic mRNA: When transcribed, the operon produces a single mRNA molecule that encodes multiple proteins (polycistronic mRNA).

• Regulatory Elements: The lacI gene encodes a repressor protein that binds to the operator to inhibit transcription.

Repression and Induction in the lac Operon

• Repression: In the absence of lactose, the lac repressor binds to the operator, blocking RNA polymerase from transcribing the operon.

• Induction: When lactose is present, it acts as an inducer by binding to the repressor, causing a conformational change that reduces the repressor's affinity for the operator. This derepresses the operon, allowing transcription.

• Negative Control: The repressor-inducer system is an example of negative control of gene expression.

Key Terms

• Operator: A DNA sequence that acts as a binding site for the repressor protein.

• Inducer: A molecule (such as allolactose) that inactivates the repressor, enabling gene expression.

• Polycistronic: Refers to mRNA that encodes multiple proteins.

Example: The lac Operon

• When E. coli is grown in the presence of lactose and absence of glucose, the lac operon is induced, leading to the production of β-galactosidase, permease, and transacetylase.

Additional info:

• The lac operon is also subject to positive regulation by the catabolite activator protein (CAP) in response to glucose levels (not shown in the provided slides).