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

Operon= a cluster of genes under control

of a single promoter


Arginine synthesis

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
  • 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