Control of Prokaryotic (Bacterial) Gene Expression
Figure 18.1 How can this fish s eyes see equally well in both air and water? Aka. Quatro ojas
Regulation of Gene Expression: Prokaryotes and eukaryotes precisely regulate gene expression in response to environmental conditions In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types RNA molecules play many roles in regulating gene expression in eukaryotes
Bacteria often respond to environmental change by regulating transcription: Natural selection has favored bacteria that produce only the products needed by that cell A cell can regulate the production of enzymes by feedback inhibition or by gene regulation One mechanism for control of gene expression in bacteria is the operon model
Feedback inhibition Precursor Regulation of a metabolic pathway trpe Enzyme 1 Enzyme 2 trpd trpc Regulation of gene expression Enzyme 3 trpb trpa Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production
Bacterial metabolism: STOP GO Bacteria need to respond quickly to changes in their environment u if they have enough of a product, need to stop production why? waste of energy to produce more how? stop production of enzymes for synthesis u if they find new food/energy source, need to utilize it quickly why? metabolism, growth, reproduction how? start production of enzymes for digestion
Remember Regulating Metabolism? Feedback inhibition - u product acts as an allosteric inhibitor of 1 st enzyme in tryptophan pathway = inhibition u but this is wasteful production of enzymes Oh, I remember this from our Metabolism Unit! -
A schematic representation of a Generalized Feedback Loop (two classifications) Negative Feedback Loop = the effect causes a reversal of the initial condition. stimulatory-inhibitory response Positive Feedback Loop = the effect intensifies initial condition. stimulatory-stimulatory response
Question? What type of Feedback Mech? stimulus What is the controlled condition? Response receptor Input Effector control center Output
Positive Feedback Mechanism
Different way to Regulate Metabolism: - Gene regulation u instead of blocking enzyme function, block transcription of genes for all enzymes in tryptophan pathway saves energy by not wasting it on unnecessary protein synthesis Now, that s a good idea from a lowly bacterium! - = inhibition -
Gene regulation in bacteria: Cells vary amount of specific enzymes by regulating gene transcription STOP u turn genes on or turn genes off GO turn genes OFF example if bacterium has enough tryptophan then it doesn t need to make enzymes used to build tryptophan turn genes ON example if bacterium encounters new sugar (energy source), like lactose, then it needs to start making enzymes used to digest lactose
Bacteria group genes together: Operon u genes grouped together with related functions example: all enzymes in a metabolic pathway u promoter = RNA polymerase binding site single promoter controls transcription of all genes in operon transcribed as one unit & a single mrna is made u operator = DNA binding site of repressor protein
So how can these genes be turned off? Repressor protein u binds to DNA at operator site u blocking RNA polymerase u blocks transcription
Operon model: RNA polymerase Operon: operator, promoter & genes they control serve as a model for gene regulation RNA polymerase TATA repressor mrna gene1 gene2 gene3 gene4 1 2 3 4 DNA promoter operator enzyme1 enzyme2 enzyme3 enzyme4 Repressor protein turns off gene by blocking RNA polymerase binding site. repressor = repressor protein
Repressible operon: tryptophan: RNA polymerase RNA polymerase TATA trp repressor Synthesis pathway model When excess tryptophan is present, it binds to tryp repressor protein & triggers repressor to bind to DNA u blocks (represses) transcription gene1 gene2 gene3 gene4 DNA mrna 1 2 3 4 trp trp promoter operator enzyme1 enzyme2 enzyme3 enzyme4 repressor repressor protein trp tryptophan trp trp trp trp trp trp trp conformational change in repressor protein! trp repressor tryptophan repressor protein complex
Tryptophan operon: What happens when tryptophan is present? Don t need to make tryptophan-building enzymes Tryptophan is allosteric regulator of repressor protein
Inducible operon: lactose RNA polymerase u induces transcription RNA polymerase TATAlac repressor gene1 gene2 gene3 gene4 mrna lac lac lac lac lac lac lac Digestive pathway model When lactose is present, binds to lac repressor protein & triggers repressor to release DNA 1 2 3 4 DNA promoter operator enzyme1 enzyme2 enzyme3 enzyme4 repressor repressor protein lac lactose conformational change in repressor protein! lac repressor lactose repressor protein complex
Lactose operon: What happens when lactose is present? Need to make lactose-digesting enzymes Lactose is allosteric regulator of repressor protein
Jacob & Monod: lac Operon 1961 1965 Francois Jacob & Jacques Monod u first to describe operon system u coined the phrase operon Jacques Monod Francois Jacob
Operon summary: Repressible operon u usually functions in anabolic pathways synthesizing end products u when end product is present in excess, cell allocates resources to other uses Inducible operon u usually functions in catabolic pathways, digesting nutrients to simpler molecules u produce enzymes only when nutrient is available cell avoids making proteins that have nothing to do, cell allocates resources to other uses
Don t be repressed! How can I induce you to ask Questions?