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Division Ave. High School Ms. Foglia AP Biology Control of Prokaryotic (Bacterial) Genes AP Biology 2007-2008 Bacterial metabolism n Bacteria need to respond quickly to changes in their environment u if they have enough of a product, need to stop production n STOP n u if they find new food/energy source, need to utilize it quickly n GO why? waste of energy to produce more how? stop production of enzymes for synthesis n why? metabolism, growth, reproduction how? start production of enzymes for digestion AP Biology Remember Regulating Metabolism? n Feedback u u inhibition product acts as an allosteric inhibitor of 1st enzyme in tryptophan pathway but this is wasteful production of enzymes - = inhibition - Oh, I remember this from our Metabolism Unit! AP Biology 1 Division Ave. High School Ms. Foglia AP Biology Different way to Regulate Metabolism n Gene regulation instead of blocking enzyme function, block transcription of genes for all enzymes in tryptophan pathway n saves energy by not wasting it on unnecessary protein synthesis u - = inhibition - Now, that’s a good idea from a lowly bacterium! AP Biology Gene regulation in bacteria n Cells vary amount of specific enzymes by regulating gene transcription u turn genes on or turn genes off turn genes OFF example if bacterium has enough tryptophan then it STOP doesn’t need to make enzymes used to build tryptophan n turn genes ON example if bacterium encounters new sugar (energy GO source), like lactose, then it needs to start making enzymes used to digest lactose n AP Biology Bacteria group genes together n Operon u genes grouped together with related functions u promoter = RNA polymerase binding site n n n u example: all enzymes in a metabolic pathway 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 AP Biology 2 Division Ave. High School Ms. Foglia AP Biology So how can these genes be turned off? n Repressor protein binds to DNA at operator site u blocking RNA polymerase u blocks transcription u AP Biology Operon model Operon: operator, promoter & genes they control serve as a model for gene regulation RNA polymerase RNA TATA repressor polymerase gene1 gene3 gene4 DNA 1 2 3 4 enzyme1 enzyme2 enzyme3 enzyme4 mRNA promoter gene2 operator Repressor protein turns off gene by blocking AP BiologyRNA polymerase binding site. repressor = repressor protein Repressible operon: tryptophan Synthesis pathway model When excess tryptophan is present, it binds to tryp repressor protein & triggers repressor to bind to DNA RNA polymerase u RNA trp repressor TATA polymerase gene2 gene3 gene4 1 2 3 4 enzyme1 enzyme2 enzyme3 enzyme4 mRNA promoter blocks (represses) transcription gene1 DNA trp operator trp trp repressor repressor protein trp trp trp trp trp trp conformational change in AP Biologyprotein! repressor trp repressor tryptophan trp tryptophan – repressor protein complex 3 Division Ave. High School Ms. Foglia AP Biology Tryptophan operon What happens when tryptophan is present? Don’t need to make tryptophan-building enzymes Tryptophan AP Biology is allosteric regulator of repressor protein Inducible operon: lactose lac lac RNA polymerase lac Digestive pathway model lac When lactose is present, binds to lac repressor protein & triggers repressor to release DNA lac lac lac RNA TATAlac repressor polymerase gene1 induces transcription gene3 gene4 1 2 3 4 enzyme1 enzyme2 enzyme3 enzyme4 mRNA promoter u gene2 operator repressor lac conformational change in AP Biologyprotein! repressor lac repressor DNA repressor protein lactose 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 AP Biology 4 Division Ave. High School Ms. Foglia AP Biology 1961 | 1965 Jacob & Monod: lac Operon n Francois Jacob & Jacques Monod first to describe operon system u coined the phrase “operon” u AP Biology Jacques Monod Francois Jacob Operon summary n Repressible operon u usually functions in anabolic pathways n u n synthesizing end products when end product is present in excess, cell allocates resources to other uses Inducible operon u usually functions in catabolic pathways, n u n AP Biology 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 5
