Advanced Microbiology Lecture 12

Advanced Microbiology: Lecture 12 - Gene Expression Regulation

• Gene expression is controlled to ensure cells only express necessary genes in specific environments.
• Gene expression regulation occurs at various levels: transcriptional, post-transcriptional, translational, and post-translational.
• Most transcriptional regulation happens at the promoter region using proteins called transcriptional regulators.
• Repressors bind to DNA operator sequences, preventing transcription initiation (negative regulation).
• Activators are needed for RNA polymerase to initiate transcription.
• Inducers increase gene expression (by activating an activator or inactivating a repressor).
• Co-repressors decrease gene expression (by activating a repressor or inactivating an activator).
• Repressible systems regulate biosynthetic pathways based on end-product availability.
• Constitutive mutants always transcribe operon genes, regardless of inducer presence.

Transcriptional Regulation Details

• Regulators can be repressors, activators, or both.
• Regulation Mechanisms: negative, positive, or both.
• Activators: bind and recruit RNA polymerase; stabilize RNA polymerase binding to the promoter; promote open complex formation; promote frequency of initiation; can also inhibit repressors.
• Repressors: simplest mechanism is steric hindrance (preventing RNA polymerase binding or proceeding); can change DNA structure, inhibiting RNA polymerase binding or proceeding; can act as anti-activators, affecting activator positive activity.

Operon Regulation

• Catabolic operons degrade compounds for building molecules.
• Biosynthetic operons synthesize molecules like amino acids, nucleotides, and vitamins.
• Negative regulation: genes are usually on (constitutive) unless a repressor turns them off. Co-repressors bind to repressors, enabling binding to the operator, thus reducing expression.
• A repressor needing a co-repressor to be active is called an aporepressor.

Positive Regulation (E. coli ara Operon)

• Operon expression depends on an activator protein presence.
• The ara operon encodes enzymes for arabinose utilization.
• AraC is a divergently encoded transcriptional activator protein with dual function.
• In the presence of arabinose, AraC changes conformation (P2 binding), activates the operon by binding to AraO2 and other operator sequences.
• The absence of arabinose causes AraC to bind to different DNA sequences (P1) preventing transcription.
• AraC has autoregulation, high concentrations prevent transcription of the operon from P1.

DNA-Binding Protein Motifs

• Helix-turn-helix (HTH) motif is a common DNA-binding motif in proteins.
• These proteins typically consist of two alpha-helices and a linker connecting them.
• Amino acids in the recognition helix (helix 2) can interact and form hydrogen bonds with DNA bases in the major groove.

This quiz covers topics related to gene expression regulation in advanced microbiology. It explores various levels of regulation, including transcriptional mechanisms, roles of activators and repressors, and the effects of inducers and co-repressors. Test your understanding of how gene expression is fine-tuned in different cellular environments.