14.2

Positive Control Mechanism

  • Positive control is mediated by an activator protein.

    • The activator protein binds to the activator site in the DNA.

    • This binding stabilizes RNA polymerase at the promoter region.

Catabolite Activator Protein (CAP)

  • Involved in the regulation of the lac operon, referred to as CAP.

  • CAP is an activator protein that, when active, binds to the CAP site located upstream of the promoter.

Promoter and Sigma Factors

  • The affinity of sigma factors for promoters is an important consideration.

    • Promoter regions have specific consensus sequences.

    • The closer a promoter is to the consensus sequence, the tighter the sigma factor binds.

  • Genes controlled by activator proteins often have promoters that differ significantly from the consensus.

    • Typically results in low affinity for the sigma factor and, consequently, RNA polymerase fails to bind without the activator protein's assistance.

Interaction with RNA Polymerase

  • The interaction is crucial for stabilizing RNA polymerase at the promoter.

  • Activator sites can be located right next to the promoter or distantly located on the DNA.

    • The distance does not hinder the interaction between the activator protein and RNA polymerase.

Role of Effector Molecules

  • Both repressors and activators are regulated by effector molecules, which influence their binding abilities.

  • CAP binding to DNA is affected by the presence of cyclic AMP, an important effector.

Cyclic AMP (cAMP)

  • Functions as an inducer in the regulation of the lac operon.

  • Commonly known as a secondary messenger in various physiological processes, particularly in eukaryotic cells.

  • Synthesized by the enzyme adenylyl cyclase, which converts ATP to cyclic AMP.

    • ATP is adenosine triphosphate, and cyclic AMP consists of one phosphate (adenosine monophosphate) cyclized with a sugar molecule.

Influence of Glucose on cAMP Levels

  • High glucose levels inhibit adenylyl cyclase activity, resulting in:

    • Low levels of cAMP because the enzyme is not active.

    • There is little cAMP binding to CAP.

  • Conversely, under low glucose conditions:

    • Adenylyl cyclase is active because there is no glucose to inhibit it.

    • High levels of cyclic AMP are produced.

    • Enough cAMP present to bind to CAP, enabling its active form to bind to the CAP site on DNA.

Scenarios: Glucose Levels and cAMP

Scenario 1: Low or Absence of Glucose

  • Adenylyl cyclase is active.

  • High levels of cyclic AMP are present.

  • cAMP can bind to CAP, changing its conformation to allow binding to the CAP site in the DNA.

Scenario 2: High Glucose Levels

  • Adenylyl cyclase is inactive due to inhibition by glucose.

  • Low levels of cAMP are present; hence:

    • cAMP cannot effectively bind to CAP.

    • CAP is in an inactive form and unable to bind to DNA.

Summary of Control Systems

  • Positive control directly involves CAP and is influenced by glucose.

  • Negative control involves repressors and responds to the presence or absence of lactose.

  • Understanding both systems is essential to determining if transcription will occur: it's crucial to integrate the effects of both positive and negative controls to forecast transcription outcomes.