Chapter 18 - Operons

1. Chapter 18 - Operons Control of Gene Expression in Prokaryotes

2. trp Sample Pathway for Control of Tryptophan (trp) Expression Promotor (RNA polymerase binding site) transcription translation Enzyme 1 Gene 1 Each enzyme catalyzes the next series of reactions necessary for tryptophan production Enzyme 2 Gene 2 Enzyme 3 Gene 3 Enzyme 4 Gene 4 Enzyme 5 Gene 5 2 Negative Feedback shut-off systems mRNA (transcription unit) DNA

3. 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

4. Basic Vocabulary Operon: - an entire gene-regulating system in PROKARYOTES, which includes the operator, promotor, and all corresponding genes

5. Basic Vocabulary 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 Let’s take a closer look at the tryptophan (trp) operon…an example of a “repressor” system

6. trp Trp operon controls the production of the amino acid tryptophan Overview of Repressor Trp Operon Components r. Trp Repressor Gene (always “on”) promotor 5 Genes needed for tryptophan production rp. RNA Pol. binding site o. Operator r. rp. o. Gene 1 Gene 1 Gene 3 Gene 4 Gene 5 RNA polymerase mRNA transcription unit mRNA for trp repressor Enz 1 Enz 2 Enz 3 Enz 4 Enz 5 Trp repressor protein translated in its “inactive” form

7. How is the trp operon turned off once enough trp is made? trp trp trp trp trp RNA pol binding site operator Trp repressor gene Inactive trp repressor protein Increased concentration of trp (co-repressor) increases chances of trp binding to allosteric site of inactive trp repressor

8. 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

9. 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.

10. Watch this animation to fully understand and review this operon, then take the on-line quiz together as a class http://bcs.whfreeman.com/thelifewire/content/chp13/1302002.html

11. 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.

12. Inducible Lac operon promotor Genes that promote ß-galactosidase production rp: RNA pol.binding site o. operator crp: helps RNA pol. to bind I. Lac Inducer I. crp. rp. o. mRNA for inducer protein RNA pol. blocked from ß-galactosidase transcription without lactose co-inducer inducer protein in “active” form

13. TWO regulatory mechanisms used to turn on lac operon • 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.

14. 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 1) Allolactose co-inducer attaches to allosteric site of inducer protein, inactivating it 2. cAMP attaches to CRP transcription factor when glucose is scarce

15. Lac Operon Animation Watch this animation to fully understand and review the lac operon! http://highered.mcgraw-hill.com/olc/dl/120080/bio27.swf

16. How are genes controlled in eukaryotes? • Regulation of Chromatin Structure • Pre and post Transcriptional Regulation • Pre and Post Translational Regulation

17. Regulation of Chromatin Structure N-terminus (amino group) of histone proteins face outwards from nucleosome Tails are thus able to be modified chemically

18. Regulation of Chromatin Structure • Histone Acetylation – neutralizes (+) charges on tails, which prevents binding to adjacent nucleosome  loose chromatin structure results, allowing for increased transcription • Methylation  Promotes condensation • Phosphorylation  can prevent condensation, if phosphorylation is adjacent to methyl group

19. 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

21. 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

22. Post-Transcriptional Regulation • Time of mRNA degradation can vary

23. 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

24. Post-Translational Regulation • During protein processing, folding • Timing of protein degredation can vary • Proteasomes degrade proteins that are tagged by ubiquitine molecules