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 controlof a single promoterRegulon?

Arginine synthesis

Mechanism of induction

Lac operon

Mechanism of induction-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

Activator protein without inducer-positive control

Activator protein with inducer

DNA binding proteins

• Non-specific, eg. histones– Small proteins, high + charge

• Specific– Frequently dimers– Interact with inverted repeats– Eg. lac repressor

DNA binding proteinsDimeric proteins (e.g., lac repressor) interact with

inverted repeats

Attenuation• 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?

• 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: leader sequence

Attenuation: delayed translation

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

Attenuation: undelayed translation

Leader peptide is formedStop codon or stem-loop structure can form in mRNAAnd transcription is attentuated

Global control: catabolite repression- a variety of unrelated genes

regulated

Diauxic growth

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

CAP binding site on the lac operon

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

• 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

2 component regulatory systems

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

Chemotaxis