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Sites of regulation. Feedback inhibition. Mechanism of allosteric inhibition. Repression and Induction. Mechanism of repression- Negative control. Mechanism of repression- control by co-repressor. Operon= a cluster of genes under control of a single promoter Regulon?. Arginine synthesis.

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Mechanism of repression-Negative control

Mechanism of repression control by co repressor
Mechanism of repression-control by co-repressor

Operon= a cluster of genes under control

of a single promoter


Arginine synthesis

Mechanism of induction negative control
Mechanism of induction-negative control

Positive vs negative control
Positive vs. Negative control

  • Repressors are “negative control”

    • An active repressor (- inducer or + corepressor) stops transcription

  • Activator proteins are “positive control”

    • The regulatory protein (activator) promotes transcription

    • Example: maltose regulon

Dna binding proteins
DNA binding proteins

  • Non-specific, eg. histones

    • Small proteins, high + charge

  • Specific

    • Frequently dimers

    • Interact with inverted repeats

    • Eg. lac repressor

Dna binding proteins dimeric proteins e g lac repressor interact with inverted repeats
DNA binding proteinsDimeric proteins (e.g., lac repressor) interact with inverted repeats


  • Positive and negative control affect initiation of transcription

  • Attenuation affects continuation of transcription

    • Eg. the tryptophan operon has a leader that includes two tryptophan residues

    • When tryptophan is lacking, the translation is delayed

    • The speed of translation determines which of two mRNA double-stranded loops form

      • One of the two possible loops is a termination signal

How does it work
How does it work?

  • Transcription and translation occurring almost simultaneously

    • Rate of transcription influenced by rate of translation

  • Translation of leader PEPTIDE regulates transcription

    • Synthesis of leader terminates transcription, and leader synthesis is inhibited by low Trp

Attenuation delayed translation
Attenuation: delayed translation

Ribosome pauses at trp codon, stem loop that forms DOES not terminate transcription

Attenuation undelayed translation
Attenuation: undelayed translation

Leader peptide is formed

Stop codon or stem-loop

structure can form in mRNA

And transcrition is attentuated

Global control catabolite repression a variety of unrelated genes regulated
Global control: catabolite repression- a variety of unrelated genes regulated

Diauxic growth

Catabolite repression
Catabolite repression

  • Catabolite activator protein (CAP) assists binding of RNA polymerase to promoter

  • CAP can bind only when it first binds cAMP

  • Adenylate cyclase: ATP -> cAMP + pyrophosphate

  • Glucose inhibits adenylate cyclase and stimulates cAMP excretion

  • Catabolite repression is similar to positive control, but the difference is the global nature of catabolite repression

Quorum sensing
Quorum sensing

  • Also a form of global control

  • Relatively recent discovery

  • AHL-acylated homoserine lactone

    • Diffusible

    • Inducer needs activator protein

  • Example, bioluminescence and luxR activator

    • Only when [AHL ] is high enough will LuxR activate the lux operon

2 component regulatory systems
2 component regulatory systems

  • Maltose=effector, BUT if signal not DIRECTLY involved, but needs to be transmitted and changed = signal transduction

  • Sensor protein=

    • kinase, phosphorylates compounds,

    • membrane associated

  • Phosphoryl group transmitted to another regulator IN the cell

    • Often a DNA binding protein involved in transcription

  • Many examples, N-fixation, sporulation,chemotaxis


  • Attractants decrease rate of autophosphorylation

  • Repellant increased autophosphorylation

  • CheA-CheW=transducer

  • CheY controls switch

    • cheY-P tumbles, CCW-CW

  • CheB phosporylated by CheA-P, but slower response than CheY-P

  • CheB involved methylation

    • Fully methylated = best for repellants

    • cheB-P demethylates, occurs when attractants High

    • Degree of methylation regulates attraction/repulsion