1. CONTROL MECHANISMS
Sections 5.5
Page 255

2. Outline
Gene regulation
Operons
lac operon
trp operon
Recap

3. Gene regulation Housekeeping genes
Always needed
constantly being transcribed/translated
Ex. Single stranded binding proteins
Most genes are not housekeeping genes

4. Gene regulation involves turning certain genes on or off
depends on whether their products are required
Regulators are the proteins that switch genes on or off. The 2 types of regulators are:
Activators (turn genes on)
Repressors (turn genes off)

5. Recall… Protein synthesis occurs in two distinct stages:
Transcription, in the _________
Product: ____
Translation, in the ______________
Product: __________

6. How do we control gene expression
4 Levels of control
Transcriptional
Posttranscriptional
Translational
Posttranslational

7. Levels (cont’d)
Transcriptional gene regulation
Regulation of which genes are transcribed
Can involve control of the rate at which transcription occurs
Posttranscriptional gene regulation
Modifications made to primary transcript in the nucleus

8. Take a break… Can you make an educated guess about:
Translational control??
Post-translational??

9. Levels (cont’d)
Translational gene regulation
Control of how often and how rapidly mRNA transcripts are translated into proteins
Posttranslational gene regulation
Modifications made to the polypeptide chain

10. Transcriptional regulation
Two classic examples:
lac operon – negative regulation
trp operon – positive regulation

11. only in prokaryotic cells
What is an operon?
a cluster of genes
all genes are under the control of one set of regulatory sequences
promoter and operator sequences
regulation is dependent on the presence or absence of effector molecules

12. Operator sequence Sequence to which regulator protein binds
Depending on the regulator, binding to the operator causes either:
activation of transcription
repression of transcription

13. Effector Acts on the regulator protein Can be an inducer
stimulates transcription
Can be a co-repressor
inhibits transcription

14. THE LAC OPERON

15. Lac operon lactose = glucose + galactose disaccharide found in milk
E. coli: ß-galactosidase catalyses the cleavage of the bond in lactose
Not economical to produce ß-galactosidase when lactose is not present

16. Lac operon (cont’d)
Negative control mechanism prevents production of ß-galactosidase if lactose is absent

17. Structure of lac operon
Cluster of genes: lacZ, lacy, lacA
Each codes a different part of the enzyme
ALL are under the control of one promoter

18. LacI protein – repressor protein
LacI protein – repressor protein. Blocks transcription of the lac operon by binding to the operator.
Physical blockage: Covers part of the lac promoter

19. Mechanism of regulation
animations/content/lacoperon.html

20. Mechanism of regulation

21. Mechanism of regulation
lactose is absent
lactose is present
LacI binds to lac promoter.
No transcription.
Lactose binds to LacI protein.
LacI changes conformation → unable to bind lac operator.
RNA polymerase can access lac promoter; transcription occurs.

22. What is the effector molecule?
_______________
It acts as an inducer – it induces transcription by deactivating the repressor protein.

23. Negative control
Default: Transcription is off, due to binding of a repressor protein.
Presence of an effector (inducer) removes the repressor from the operator region.
Transcription can now occur.

24. TRP OPERON

25. The trp operon Tryptophan – Amino acid
Operon: cluster of five genes encoding enzymes required for trp synthesis
Transcription of operon is repressed when concentrations of trp in the cell are high

26. Positive control mechanism prevents production of tryptophan,
trp operon (cont’d)
Positive control mechanism prevents production of tryptophan,
if tryptophan levels are high
hill.com/olc/dl/120080/bio26.swf

27. Mechanism of regulation

28. Mechanism of regulation
trp absent
trp present
trp operon is transcribed (default)
trp itself will bind to the repressor protein.
conformational shape occurs
allows repressor to bind to the trp operator → shuts off transcription

29. What is the effector molecule?
_______________
It acts as a co-repressor. It binds to a repressor to activate it in order to repress transcription.

30. Positive control Default: Transcription is on
Presence of an effector (co-repressor) causes binding of the repressor to the operator.
Transcription does not occur.

31. trp vs. lac operons
Default state
Effector
Positive control
Transcription is on
Allows repressor to bind
Negative control
Transcription is off
Removes repressor
Positive vs. Negative Control: Is the repressor added or taken away?

32. Recap: Vocabulary Basic terms:
Gene regulation: Describes any process that alters the rate of gene expression.
Operon: A cluster of closely-related genes. They are all controlled by one set of regulatory sequences.

33. Types of molecules involved:
Regulators: Molecules that carry out translational gene regulation.
Classified as either activators, or repressors.
Example: LacI repressor; trp repressor
Effectors: Substances to which regulator proteins respond.
Inducers or Co-repressors
Example: Lactose, Tryptophan

34. Regulatory DNA sequences:
Promoter: DNA sequence to which RNA polymerase binds to begin transcription.
Operator: DNA sequence to which a transcription factor binds in order to alter transcription. Close to the promoter.

35. Homework
Pg. 258 #1-6