Gene Regulation in Bacteria

Gene Regulation in Bacteria

• Bacteria control gene expression to conserve energy and resources, avoid contradictory functions, and respond to their environment. This regulation occurs at multiple levels, including DNA sequence alteration, transcription control, mRNA stability, translational control, and post-translational control.

Transcriptional Regulation

• Transcription initiation is a major point of control, often managed by regulatory proteins that bind to DNA near the gene promoter.
• Regulatory proteins bind DNA at or near gene promoters, influencing whether RNA polymerase binds to the promoter. They often bind as dimers and interact with the DNA's major groove.
• DNA targets frequently exhibit inverted repeats, allowing dimeric binding.

Repressors and Activators

• Repressors prevent gene expression by binding operator sequences in the DNA. Some bind always, while others require a ligand (corepressor).
• Activators stimulate gene expression. They bind poorly to DNA until bound to an inducer, then interact directly with RNA polymerase, promoting transcription.

Two-Component Signal Transduction

• This system helps microbes sense the environment using:
  • Sensor kinase: A membrane-bound protein that detects environmental signal, phosphorylates itself, and then passes phosphate to a target.
  • Response regulator: A cytosolic protein that receives the phosphate from the sensor kinase, and then binds to DNA, either promoting or repressing gene expression. Phosphate is removed by a phosphatase to deactivate the system.

The Lactose Operon (lac Operon)

• The lac operon in E. coli is a model system for understanding inducible gene expression.
• The lac operon controls enzymes involved in lactose metabolism.
  • LacY (Lactose permease): Transports lactose into the cell.
  • LacZ (β-Galactosidase): Breaks down lactose into simpler sugars, or modifies it to allolactose.
• Regulation:
  • Lactose absence: The lac repressor (LacI) blocks transcription by binding to the operator.
  • Lactose presence: Lactose is converted to allolactose, which binds to and inactivates the repressor. This allows RNA polymerase to transcribe the operon.
• cAMP and CRP: cAMP levels increase when energy sources are scarce and bind to cAMP receptor protein (CRP). CRP, then binds to the DNA increasing expression of the lac operon.

Catabolite Repression

• Catabolite repression occurs when a readily available nutrient like glucose inhibits the expression of genes required for metabolizing other nutrients like lactose.
• Glucose presence: Prevents high-level expression of genes needed to metabolize lactose. Glucose transport inhibits lactose entry and prevents lac operon induction.
• Glucose absence: Allows lactose transport, and lac operon expression. This leads to a diauxic growth curve when two carbon sources are present.

Explore the mechanisms of gene regulation in bacteria, focusing on transcriptional control and the role of regulatory proteins. Learn how repressors and activators influence gene expression and the strategies bacteria use to manage their genetic resources efficiently.