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Control of Prokaryotic (Bacterial) Genes
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Bacterial metabolism Bacteria need to respond quickly to changes in their environment 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 if they find new food/energy source, need to utilize it quickly why? metabolism, growth, reproduction how? start production of enzymes for digestion STOP GO
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Regulating Metabolism
Feedback inhibition product acts as an allosteric inhibitor of 1st enzyme in tryptophan pathway Tryptophan STOPS the pathway by binding to Enzyme 1 Allosteric means a protein has two forms: active and inactive (will get to this later) but this is wasteful production of enzymes
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Different way to Regulate Metabolism
Gene regulation 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
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Gene regulation in bacteria
Cells vary amount of specific enzymes by regulating gene transcription turn genes on or turn genes off turn genes OFF example if bacterium has enough tryptophan then it doesn’t need to make enzymes used to build tryptophan (this gene “set” is normally turned on) turn genes ON example if bacterium encounters new sugar (energy source), like lactose, then it needs to start making enzymes used to digest lactose (this gene “set” is normally turned off) STOP Remember: rapid growth generation every ~20 minutes 108 (100 million) colony overnight! Anybody that can put more energy to growth & reproduction takes over the toilet. An individual bacterium, locked into the genome that it has inherited, can cope with environmental fluctuations by exerting metabolic control. First, cells vary the number of specific enzyme molecules by regulating gene expression. Second, cells adjust the activity of enzymes already present (for example, by feedback inhibition). GO
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Bacteria group genes together
Operon genes grouped together with related functions example: all enzymes in a metabolic pathway promoter = RNA polymerase binding site single promoter controls transcription of all genes in operon transcribed as one unit & a single mRNA is made operator = DNA binding site of repressor protein These 3 parts make up the Operon Let’s chat a bit about this CRAZY picture!
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So how can these genes be turned off?
Repressor protein Remember if the bacteria has enough tryptophan it doesn’t need to keep making more binds to DNA at operator site blocking RNA polymerase blocks transcription A regulatory gene (upstream from the operon) on the DNA codes for the repressor protein
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Operon model promoter operator
Operon: operator, promoter & genes they control serve as a model for gene regulation RNA polymerase RNA polymerase repressor gene1 gene2 gene3 gene4 DNA promoter operator 1 2 3 4 mRNA enzyme1 enzyme2 enzyme3 enzyme4 Repressor protein turns off gene by blocking RNA polymerase binding site. repressor = repressor protein
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Tryptophan operon Synthesis Pathway Model: Making Tryptophan
Bacteria need tryptophan to survive, transcription of gene is normally “on” Environments not usually tryptophan-rich What happens when tryptophan is present? Don’t need to make tryptophan-building enzymes Need to shut it off
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Repressible Operon: Tryptophan
When excess tryptophan is present, it binds to trp repressor protein & triggers repressor to bind to DNA blocks (represses) transcription RNA polymerase RNA polymerase repressor trp gene1 gene2 gene3 gene4 DNA promoter operator 1 2 3 4 mRNA trp trp enzyme1 enzyme2 enzyme3 enzyme4 trp trp trp trp repressor repressor protein trp trp trp tryptophan trp conformational change in repressor protein! repressor tryptophan – repressor protein complex trp
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Table Summary Name of Operon Trp Operon
Regulatory gene (located away from operon) trpR (REPRESSIBLE) # of genes in operon 5 genes (5 enzymes) Allosteric protein (represser) Protein in INACTIVE state (doesn’t bind to operator—transcription occurs) When “specific” molecule is present *IMPORTANT CONCEPT!!! When tryptophan is PRESENT, it makes the repressor protein “active”, therefore able to bind to operator and stops transcription for the gene tryptophan—no unnecessary waste of resources Name of specific molecule Tryptophan
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Lactose operon Digestive Pathway Model: Breaking down lactose
Bacteria aren’t always exposed to lactose Enzymes for breaking down lactose are normally turned off What happens when lactose is present? If we drink some milk, then E.Coli need to be able to break it down (lactose=glucose & galactose) Need to make lactose-digesting enzymes
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Inducible Operon: Lactose
When lactose is present, binds to lac repressor protein & triggers repressor to release DNA induces transcription RNA polymerase RNA polymerase repressor TATA lac gene1 gene2 gene3 gene4 DNA promoter operator 1 2 3 4 mRNA enzyme1 enzyme2 enzyme3 enzyme4 repressor repressor protein lactose lac conformational change in repressor protein! repressor lactose – repressor protein complex lac
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Positive Gene Regulation: A layer to add to the lactose operon
Bacteria prefer glucose over lactose Enzymes in glycolsis are always present unlike the enzymes to breakdown lactose (which are found in the lac operon) Once glucose has decreased then lactose is used as an energy source How does a bacteria cell sense/know glucose concentrations? cAMP is on the case Do you remember “him” from cell communication and the G-protein-coupled receptor (GPCR)
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Positive Gene Regulation: Glucose is present
Glucose is present in high concentrations—cAMP low levels CAP-binding site “open” CAP protein activator is synthesized in its inactive form RNA polymerase doesn’t really want to bind (sitting stupid)-even with the operator “open”
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Positive Gene Regulation: Glucose in low concentration
cAMP will accumulate when glucose has decreased CAP (protein activator is synthesized in its inactive form) becomes active when cAMP binds to a portion of the protein Once activated, CAP w/cAMP will directly bind to the CAP site on the DNA promoter This stimulates gene expression Now lactose genes can be transcribed
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Table Summary Name of Operon Lac Operon
Regulatory gene (located away from operon) lacI INDUCIBLE # of genes in operon 3 genes (3 enzymes) Allosteric protein (represser) Protein in ACTIVE state (binds to operator-shutting off transcription) When “specific” molecule is present *IMPORTANT CONCEPT!!! When lactose if PRESENT it makes the repressor protein “inactive”, therefore no longer able to bind to operator and transcription occurs, enzymes made—able to breakdown lactose Name of specific molecule Allolactose (isomer of lactose)
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Operon summary Repressible operon Inducible operon
usually functions in anabolic pathways synthesizing end products when end product is present in excess, cell allocates resources to other uses Inducible operon usually functions in catabolic pathways, digesting nutrients to simpler molecules produce enzymes only when nutrient is available cell avoids making proteins that have nothing to do, cell allocates resources to other uses
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So where are all these OPERONs at?
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