Lecture 11 - Coordination of Cell Processes

Describe the levels by which the concentration of a protein can be regulated.

Transcriptional Level

mRNA Level

Translation Level

Post-translational Level

Examples of Transcriptional Level Regulation

Frequency of RNAP recruitment to a promoter

leve lf DNA supercoiling affects accessibility

Examples of mRNA Level Regulation

Frequency of transcription termination

mRNA stability (short lived in microbes)

Examples of Translational Level Regulation

Frequency of ribosome assembly onto mRNA

Examples of Post-translational Level Regulation

Protein Localization

Degradation or stabilization of proteins

Competitive Inhibition

inhibitor molecules resemble the substrate and compete for the enzyme's active site., preventing the real substrate from binding.

Non-competitive Inhibition (allosteric)

inhibitor binds to a site other than the active site (an allosteric site), changing the enzyme's shape, preventing the substrate from binding onto the enzyme

Covalent Modification

enzyme activity can be altered by chemical modifications, causing reversible changes that "switch" activity on or off.

Examples of Covalent Modification

Phosphorylation

Methylation

Acetylation

Sigma Factors

proteins in prokaryotic cells that bind to RNA polymerase and direct it to specific classes of promoters

Explain differences between various sigma factors.

different sigma factors recognize different sequences, controlling transciption of the genes and allowing bacteria to switch transcriptional programs depending on conditions (e.g., stress, starvation, heat shock)

Lac Operon

a gene system whose operator gene and three structural genes control lactose metabolism in E. coli

Cis-acting elements

DNA sites that do not move

Examples of Cis-acting Elements

Promoter

Operator

CRP-cAMP binding site

Promoter

the site where RNA polymerase binds, activating transcription

Operator

a site on the lac operon for LacI (and inhibitor) to bind onto, blocking RNA polymerase from transcription

CRP-cAMP binding site

a site on the lac operon where CRP-cAMP binds to enhance transciption

Trans-acting Factors

a regulatory protein that binds to a regulatory element in the DNA and exerts a trans effect

Examples of Trans-acting factors

LacI repressor protein

Allolactose (an inducer)

CRP-cAMP

LacI repressor protein

binds operator and blocks transcription when lactose is absent

Allolactose (an inducer)

binds to LacI and prevents it from repressing, allowing transcription

CRP-cAMP

bind the promoter region to increase RNAP binding when glucose is low

Is the lac operon ever off?

No, the lac operon is always on to produce β-galactosidase, converting lactose into allollactose to be used.

Feedback Inhibition

A method of metabolic control in which the end product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway.

Example of a feedback inhibition mechanism

Histidine biosynthetic pathway

Histidine Biosynthesis Pathway

excess histidine inhibits the first enzyme in the pathway, preventing unnecessary synthesis.