How do Bacteria Control Gene Expression?

1. How do Bacteria Control Gene Expression? • Individual bacteria respond to environmental change by regulating their gene expression • A bacterium can ADJUST its metabolism to the changing environment and food sources • This metabolic control occurs on 2 levels: • Adjusting activity of enzymes (fig. 18.20a) • Regulating genes that encode enzymes (fig. 18.20b)

2. Regulation of enzyme production Regulation of enzyme activity Precursor Feedback inhibition Enzyme 1 Gene 1 LE 18-20 Gene 2 Enzyme 2 Regulation of gene expression Gene 3 Enzyme 3 Enzyme 4 Gene 4 Gene 5 Enzyme 5 Tryptophan

3. Operons: The Basic Concept • Only in bacteria  genes that work together to regulate some function (ie. production of tryptophan) are often clustered (grouped) into operons • INCLUDES: • An operator- an “on-off” switch. Positioned on or just after promoter. • A promoter with a TATA box • Genes encoding metabolic enzymes (ie. enzymes needed to produce tryptophan, or to break down lactose)

4. Operons: The Basic Concept Continued • An operon can be switched off by a protein called a repressor  binds to operator, blocking RNA polymerase so it can’t bind to promoter  What effect do you think this has? • A corepressor is a small molecule that cooperates with a repressor to switch operon off. Repressor is inactive  corepressor binds to repressor protein repressor protein changes shape to become ACTIVATED • http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_3.html

5. Operon Parts • The regulatory genes code for the repressor protein (located some distance away from operon it controls). • The promoter site is the attachment site for RNA polymerase (preceededby TATA box) • The operator site is the attachment site for the repressor protein. • The structural genes code for the enzymes that direct the production of an end product (ie. tryptophan) or the breakdown of another molecule (ie. lactose). Basically, these genes code for the enzymes that regulate the metabolic process at hand.

6. Operon Parts Continued • The regulatory metabolite is either the product of the reaction (ie. tryptophan) or the reactant (ie. lactose) depending on the type of operon. • The repressor [protein] is different for each operon and has a binding site to custom fit to the regulatory metabolite. Prevents transcription of the genes.

7. Operon- Example #1 • trp operon- a repressible operon • Used to MAKE tryptophan (amino acid) • Promoter (and TATA box): RNA polymerase binding site; begins transcription • operator: controls access of RNA polymerase to genes (EMPTY when tryptophan NOT present) • repressor: protein that binds to operator and prevents attachment of RNA polymerase • coded from a regulatory gene (when tryptophan is present, tryptophan acts as a corepressor) • transcription is repressedwhen tryptophan binds to a regulatory (repressor) protein

8. NO Tryptophan present repressor Inactive  operon ON trp operon TATA Box and Promoter Structural Genes of operon DNA trpB trpA trpE trpC trpD trpR Operator Stop codon RNA polymerase Regulatory gene 3¢ mRNA 5¢ mRNA Start codon 5¢ D B E C A Inactive repressor Protein Polypeptides that make up Enzymes needed to make tryptophan

9. DNA LE 18-21b_1 mRNA Active repressor Protein Tryptophan (corepressor) Tryptophan present  repressor active  operon OFF

10. DNA No RNA made LE 18-21b_2 mRNA Active repressor Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off

11. Tryptophan Repressor Operon • http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html# • Animation #1

12. Operon- Example #2 • lac operon- an inducible operon • lactose breakdown (assume NO glucose in habitat) • When lactose not present: • repressor active • operon off • no transcription for lactose breakdown enzymes • When lactose present: • repressor inactive • operon on • inducer molecule (allolactose) inactivates protein repressor • transcription is stimulated when inducer binds to a regulatory protein • http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html

13. Recall…What are Glucose and Lactose? • Glucose • Monosaccharide • Needed for bacterial glycolysis to make ATP • Lactose • Disaccharide • made of glucose and galactose

14. Promoter Regulatory gene Operator lacl lacZ DNA LE 18-22a No RNA made 3¢ mRNA RNA polymerase 5¢ Active repressor Protein Lactose absent  repressor active  operon OFF

15. lac operon DNA lacl lacY lacA lacZ LE 18-22b RNA polymerase 3¢ mRNA mRNA 5¢ 5¢ Transacetylase Permease -Galactosidase Protein Enzymes needed for lactose metabolism Inactive repressor Allolactose (inducer) Lactose present  repressor inactive  operon ON

16. Repressible and Inducible Operons: 2 Types of Negative Gene Regulation • A repressible operon is usually on • binding of a repressor to operator shuts off transcription • The trp operon is a repressible operon • An inducible operon is usually off • a molecule called an inducer inactivates the repressor and turns on transcription • Example of an inducible operon is the lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose

17. Inducible enzymes usually function in catabolic pathways • Repressible enzymes usually function in anabolic pathways • Regulation of the trp and lac operons involves negativecontrol of genes because operons are switched off by the active form of the repressor

18. Positive Gene Regulation • Some operons are also subject to positive control through a stimulatory activator protein, such as catabolite activator protein (CAP) • When glucose (a preferred food source of E. coli ) is scarce, the lac operon is activated by the binding of CAP • When glucose levels increase, CAP detaches from the lac operon, turning it off

19. Promoter DNA lacl lacZ LE 18-23a RNA polymerase can bind and transcribe Operator CAP-binding site Active CAP cAMP Inactive lac repressor Inactive CAP Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

20. Promoter DNA lacl lacZ CAP-binding site Operator RNA polymerase can’t bind Inactive CAP Inactive lac repressor Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

21. VERY GOOD interactive • http://uccpbank.k12hsn.org/courses/APBioI/course%20files/multimedia/chapter8exploration05/lessonp.html

22. Lac Operon (with and without lactose/ glucose) • http://wps.prenhall.com/wps/media/objects/487/499061/CDA14_1/CDA14_1b/CDA14_1b.htm • http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html# • http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_4.html • http://www.dartmouth.edu/~cbbc/courses/movies/LacOperon.html • http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_3.html