Lac Operon


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Lac Operon

  1. 1. Sample Pathway for Control of Promotor (RNA Tryptophan (trp) Expression polymerase binding site) Enzyme 1 Gene 1 Enzyme 2 Gene 2 Enzyme 3 DNA Gene 4 not a s na t i l r not pr cs na t i i r Gene 3 Gene 5 mRNA (transcription unit) Enzyme 4 Each enzyme catalyzes the next series of reactions necessary for tryptophan production Enzyme 5 trp 2 Negative Feedback shut-off systems
  2. 2. Basic Vocabulary mRNA Transcription Unit: - grouping of mRNA genes with related function - 2 advantages ~ 1. organization 2. easy regulation – one “on/off” switch can be used to control an entire cluster of related genes
  3. 3. Basic Vocabulary Operon: - an entire gene-regulating system in PROKARYOTES, which includes the operator, promotor, and all corresponding genes
  4. 4. Basic Vocabulary Let’s take a closer look at the tryptophan (trp) operon…an example of a “repressor” system Operator: - “on-off” switch to genes - controls the access of RNA polymerase to genes - found within the promotor region or between the promoter and gene-encoding regions
  5. 5. Overview controls the Trp Operon Trp operonof Repressor production Components of the amino acid tryptophan r. Trp promotor Repressor 5 Genes needed for Gene tryptophan production rp. RNA Pol. o. Operator (always binding site “on”) r. rp. o. Gene 1 Gene 1Gene 3 Gene 4 Gene 5 RNA polymerase mRNA transcription unit mRNA for trp repressor Enz 1 Trp repressor protein translated in its “inactive” form Enz 2 Enz 3 Enz 4 Enz 5 trp
  6. 6. How is the trp operon turned off once enough trp is made? Trp repressor gene RNA pol binding site operator trp trp trp trp Inactive trp repressor protein trp Increased concentration of trp (corepressor) increases chances of trp binding to allosteric site of inactive trp repressor
  7. 7. How is the trp operon turned off once enough trp is made? RNA polymerse, therefore is physically blocked from transcribing genes for trp Active trp repressor can now bind to operator
  8. 8. Why is this a repressor system? The trp operon is a repressor operon, meaning gene expression of the operon is repressed by the presence of the co-repressor, tryptophan.
  9. 9. Watch this animation to fully understand and review this operon, then take the on-line quiz together as a class
  10. 10. Overview of Inducible Lac Operon The Lac operon controls the production of the ß-galactosidase, an enzyme that catalyzes the hydrolysis (break-down) of lactose into glucose and galactose. This is an inducible operon, meaning gene expression ß-galactosidase is stimulated by the presence of an co-inducer, lactose.
  11. 11. Inducible Lac operon promotor I. Lac Inducer I. mRNA for inducer protein inducer protein in “active” form crp: helps RNA pol. to bind rp: RNA pol.binding site crp. rp. o. operator Genes that promote ßgalactosidase production o. RNA pol. blocked from ßgalactosidase transcription without lactose co-inducer
  12. 12. TWO regulatory mechanisms used to turn on lac operon 1) Presence of lactose as a co-inducer ** reason? ß-galactosidase is not needed unless lactose needs to be broken down 2) Low amounts of glucose ** reason? Recall that lactose breaks down into glucose and galactose. Low glucose levels signals the cell for more lactose to be broken down. Both conditions must be met for the lac operon to turn on.
  13. 13. TWO regulatory mechanisms used to turn on lac operon RNA polymerase can bind only with the help of CRP transcription factor RNA pol. AlloLactose cAMP CRP Transcription Factor 2. cAMP attaches to CRP transcription factor when glucose is scarce 1) Allolactose coinducer attaches to allosteric site of induce protein, inactivating it
  14. 14. Lac Operon Animation Watch this animation to fully understand and review the lac operon!
  15. 15. How are genes controlled in eukaryotes? 1) Regulation of Chromatin Structure 2) Pre and post Transcriptional Regulation 3) Pre and Post Translational Regulation
  16. 16. Regulation of Chromatin Structure N-terminus (amino group) of histone proteins face outwards from nucleosome Tails are thus able to be modified chemically
  17. 17. Regulation of Chromatin Structure Histone Acetylation – neutralizes (+) charges on •• Methylation  Promotes condensation tails, which prevents binding to condensation, if • Phosphorylation  can prevent adjacent nucleosome  loose chromatin structure phosphorylation is adjacent to methyl group results, allowing for increased transcription
  18. 18. Pre-Transcriptional Regulation • Similar to methods used in bacterial operons, using proteins that inhibit or promote binding of RNA pol. – Distal and Proximal Control Elements – Proteins involved include: • Transcription factors • Activators • Mediator Proteins
  19. 19. Post-Transcriptional Regulation RNA Processing – differential/alternative splicing can produce different 20 mRNA transcript Differential splicing redefines which RNA segments are considered introns and which are exons
  20. 20. Post-Transcriptional Regulation • Time of mRNA degradation can vary
  21. 21. Pre-Translational Regulation Initiation of Translation - can be blocked by regulatory proteins that prevent ribosome binding - shortened polyA tails in mRNA prevents translation (polyA tails can be added during appropriate time) - global regulatory control of all mRNAs in cell
  22. 22. Post-Translational Regulation • During protein processing, folding • Timing of protein degredation can vary • Proteasomes degrade proteins that are tagged by ubiquitine molecules