Bacterial Gene Regulation: Operons and Metabolism Control

1. Ch 18 (Pt A): Control of Prokaryotic (Bacterial) Genes

2. Recommended Videos & Lectures • 1- Bozeman- Operon • 2- Several good lectures for Ch 18 (Regulation of Gene Expression) on www.youtube.com • Rocketsgeneralbio • Science with Mr J • BleierBiology

3. 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 *Recall: Prokaryotes don’t have introns or nuclear membrane. They can transcribe and translate simultaneously(Figure 17.24)

4. - - = inhibition Remember Regulating Metabolism? • Feedback inhibition • product acts as an allosteric inhibitor of 1st enzyme in tryptophan pathway (feedback inhibition-negative control) • but this is wasteful production of enzymes Oh, I remember thisfrom our Metabolism Unit!

5. - - - = inhibition Different way to Regulate Metabolism • Gene regulation- attranscriptional level • 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!

6. Gene regulation in bacteria • Cells vary amount of specific enzymes by regulating gene transcription • turn genes on or turn genes off • turn genes OFF exampleif bacterium has enough tryptophan then it doesn’t need to make enzymes used to build tryptophan • turn genes ON exampleif bacterium encounters new sugar (energy source), like lactose, then it needs to start making enzymes used to digest lactose STOP GO

7. 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(works like a ‘switch’) • *Note-Gene that codes for repressor is not part of Operon

8. So how can these genes be turned off? • Repressor protein (*Regulatory gene that codes for Repressor is outside of Operon: Shown as purple here) • Regulatory gene is always on- makes repressor • binds to DNA at operator site • blocks RNA polymerase, so it blocks transcription

9. RNA polymerase RNA polymerase repressor repressor enzyme1 1 enzyme2 2 enzyme3 3 enzyme4 4 promoter = repressor protein operator Operon model Operon: operator, promoter & genes they control serve as a model for gene regulation gene1 gene2 gene3 gene4 TATA DNA mRNA Repressor protein turns off gene by blocking RNA polymerase binding site. Made by a regulatory gene outside of promoter (not shown here)

10. Repressible Operon • For metabolic pathways that are normally ‘on’ • Ex- Trp Operon: transcribes genes for making enzymes needed to synthesize tryptophan • If tryptophan is already present in the environment, there’s no need to make enzymes for its production • Transcription can be repressed

11. trp RNA polymerase RNA polymerase repressor repressor repressor enzyme1 1 enzyme2 2 enzyme3 3 enzyme4 4 promoter repressor protein operator tryptophan trp trp trp trp trp trp trp trp trp tryptophan – repressor protein complex Repressibleoperon(inactive repressor when its alone): tryptophan Synthesis (anabolism) pathway model: When excess tryptophan is present, it binds to tryp repressor protein & triggers repressor to bind to DNA • blocks (represses) transcription gene1 gene2 gene3 gene4 TATA DNA mRNA trp conformational change in repressor protein! trp

12. Tryptophan operon What happens when tryptophan is present? Don’t need to make tryptophan-building enzymes Tryptophan is allosteric regulator of repressor protein

13. Inducible Operon • For metabolic pathways that are usually “off” • Ex: Lac Operon- makes enzymes that digest lactose. • If there’s no lactose in the environment, would be waste of energy to make enzymes to digest something that’s not there! • If there is lactose in environment, the operon can be induced to transcribe & manufacture the necessary enzymes for digestion

14. RNA polymerase RNA polymerase repressor repressor repressor enzyme1 1 enzyme2 2 enzyme3 3 enzyme4 4 promoter repressor protein operator lactose lac lac lac lac lac lac lac lactose – repressor protein complex Inducible operon: lactose (*the repressor is active when its alone) Digestive (catabolism) pathway model When lactose is present, binds to lac repressor protein & triggers repressor to release DNA • The presence of lactose induces transcription lac gene1 gene2 gene3 gene4 TATA DNA mRNA lac conformational change in repressor protein! lac

15. Lactose operon What happens when lactose is present? Need to make lactose-digesting enzymes Lactose is allosteric regulator of repressor protein

16. Positive Gene Regulation • What if the bacteria needs to ‘turn-up’ transcription? (Let’s say it’s desperate for energy and needs to digest lactose at a ‘higher rate’ to continue respiration) • A signaling pathway can stimulate activators (positive feedback) • Recall Chap. 11- Get ready…we’re going to put some concepts together! • Open 11th edition to Fig 11.12 (p.224) & then Fig 11.15. What’s happening?

17. Positive Gene Regulation • From Ch 11: • Signal/Reception (conformational change)  Transduction (secondary messenger, such as AMP, initiates phosphorylation & final product causes inactive transcription factor to become active)  Cell Response (initiates transcription!) • Now let’s continue with how activators affect prokaryotic transcription

18. Lac Operon – (11th edition: p.367; 8th edition p.355) • Lactose is ONLY used as energy source when glucose is not present in large quantities • There’s another gene outside operon that always codes for CAP protein (inactive repressor alone). *CAP- catabolic activator protein • When lactose is present & glucose is scarce, cAMP levels are high. High cAMP activates CAP protein. It binds to promoter (along w/ RNA Poly) & acts as an Activator, increasing transcription • HW- Explain what happens if glucose levels are high. I will call on someone to explain this tomorrow!

19. 1961 | 1965 Jacob & Monod: lac Operon • Francois Jacob & Jacques Monod • first to describe operon system • coined the phrase “operon” Jacques Monod Francois Jacob

20. Operon summary • Repressible operon - Ex: Trp • usually functions in anabolic pathways • synthesizing end products • when end product is present in excess,cell allocates resources to other uses • Inducible operon – Ex: Lac • 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

21. Don’t be repressed! How can I induce youto ask Questions?