1. AP Biology Chapter 13: Gene Regulation

2. Gene Regulation in Bacteria and Eukaryotes
Bacterial cells
Genetic Organization?
Grow rapidly and have short life span
Controlling transcription is the most economical way for the cell to regulate gene expression
Eukaryote cells
Long life span/respond to many different stimuli
A single gene is regulated in different ways in different types of cells
Gene regulation complex
Although transcriptional-level control predominates, control at other levels of gene expression is also important

3. Gene Regulation in Bacteria
Prokaryotic DNA is organized into units called operons, which contain functionally related genes
Operons regulated as units, so functionally related proteins are synthesized simultaneously only when needed
Each operon consists of:
Regulatory gene, controls transcription of other genes
Promoter, RNA polymerase recognizes as place to start transcribing
Operator, governs access of RNA polymerase to promoter
Structural genes, encode for related proteins

4. Inducible, Repressible, and Constitutive Genes in Bacteria
Inducible operon
Normally turned off
Catabolic pathways
Repressible operon
Normally turned on
Operate via feedback inhibition
Anabolic pathways
Constitutive genes
Constantly needed and therefore constantly transcribed
Examples: Ribosomal proteins, tRNAs, RNA polymerase, glycolysis enzymes
Neither inducible nor repressible and active at all times
The activity of constitutive genes is controlled by how efficiently RNA polymerase binds to their promoter regions.

5. Inducible Operons: lac operon
Intestinal bacterium Escherichia coli (E.coli) lives on what its host eats
Specific enzymes are needed to metabolize the type of food that comes along
e.g. in newborn mammals, E.coli are bathed in milk, containing the milk sugar lactose
The lactose operon contains three structural genes, each coding for an enzyme that aids in lactose metabolism
lactose not present: repressor active, operon off; no transcription for lactose enzymes
lactose present: repressor inactive, operon on; inducer molecule inactivates protein repressor (allolactose)
transcription is stimulated when inducer binds to a regulatory protein
Lac Operon Video

8. Allolactose bound to repressor
Lac Operon Video
Allolactose bound to repressor

9. Repressible Operons: trp Operon
Trp Operon Video
Tryptophan synthesis (anabolism)
Promoter: RNA polymerase binding site; begins transcription
Operator: controls access of RNA polymerase to genes (tryptophan not present)
Repressor: binds to operator preventing attachment of RNA polymerase ~ when tryptophan is present ~ acts as a co-repressor)
Transcription is repressed when tryptophan binds to a regulatory protein

10. Negative Control in the Regulation of an Operon
Negative regulators inhibit transcription
Repressible and inducible operons are under negative control
When repressor protein binds to operator, transcription is turned off
Seen in lac and trp operons

11. Positive Control in the Regulation of an Operon
Positive regulators stimulate transcription
Some inducible operons (lac) are also under positive control
A separate protein binds to DNA and stimulates transcription of the gene
Positive control of lac operon requires that the cell is able to sense the presence of glucose
More efficient for cell to utilize glucose before lactose
Only when lactose is present and glucose is in short supply does E.coli use lactose as energy source

12. Positive Regulation of lac Operon
Lac operon always has low affinity for RNA polymerase
Involves Two Proteins:
CAP (catabolite activator protein)
cAMP (cyclic AMP)
Together, CAP and cAMP cause RNA polymerase to bind tightly to promoter region
Levels of cAMP increase as levels of glucose decrease
Lac operon is fully active only when lactose is available and glucose levels are low

13. A Regulon
Group of functionally related operons controlled by a common regulator
Example: CAP regulates the catabolism of lactose, galactose, arabinose and maltose

14. Comprehension Check
Match these components of the lac operon with their functions.
______ b-galactosidase A. is inactivated when attached to lactose
______ cAMP-CAP complex B. codes for synthesis of
repressor
______ lactose C. hydrolyzes lactose
______ operator D. stimulates gene
expression
______ promoter E. repressor attaches here
______ regulator gene F. RNA polymerase attaches
here
______ repressor G. acts as inducer that
inactivates repressor
______ structural gene H. codes for an enzyme C D G E F B A H

15. Comprehension Check
Listed below are characteristics of repressible and inducible enzymes. Identify each of the following as true of repressible or inducible enzymes.
______ genes are switched off until a specific metabolite inactivates the repressor
______ genes are switched on until a specific metabolite activates the repressor
______ Generally function in anabolic pathways
______ Usually function in catabolic pathways
______ Pathway end product switches off its own production
______ Enzyme synthesis is switched on by the nutrient in used in the pathway
Inducible
Repressible
Repressible
Inducible
Repressible
Inducible

16. Comprehension Check ______ Allolactose binds to repressor
The events listed below describe how the lac operon functions when lactose is present and glucose is absent. Put the steps in the correct order.
______ Allolactose binds to repressor
______ cAMP accumulates
______ cAMP activates CAP
______ cAMP binds to CAP
______ cAMP/CAP complex binds to CAP site in promoter
______ CAP concentration increases
______ Genes transcribed
______ Repressor inactivated
______ RNA polymerase binds to promoter 1 4 6 5 7 3 9 2 8

17. Eukaryotic Gene Expression
Not organized into operons
Typical human cell only expresses about 20% of its genes at any given time
Remember: All body cells contain identical genome
Cells rely on differential gene expression
Regulation allows cell differentiation and organization into tissues/organs
Each gene has regulatory sequences essential to the control of transcription

18. Gene Regulation in Eukaryotic Cells
Gene regulation occurs at the levels of
Transcription
mRNA processing
Translation
The protein product

20. Eukaryotic Promoters Vary in Efficiency, Depending on UPE’s
Like prokaryotes, transcription requires an initiation and promoter sites
Eukaryotic Promoter consists of:
RNA Polymerase-binding Site (TATA box)
Upstream Promoter Elements (UPE’s)
8-12 bases upstream from TATA box
Types/Number of UPE’s determine efficiency of promoter
UPE’s required for accurate and efficient initiation of transcription
In addition to UPE’s, eukaryotic genes also controlled by Enhancers
Enhancers facilitate RNA polymerase binding to promoter
Increase rate of transcription

21. Regulation of Transcription in Eukaryotes

22. Eukaryotic Regulatory Proteins
Called “Transcription Factors”
Similar to repressors and CAP’s in prokaryotes
Usually act as activators
Enhancers only become functional when bound to specific transcription factors

23. Chromosome Organization may Affect Gene Expression
Genes are inactivated by changes in chromosome structure
Heterochromatin is tightly wound and not transcribed (ex. Barr body)
Euchromatin is loosely packed and easily transcribed
DNA methylation
Methyl groups added to cytosines
Make transcription impossible
Multiple copies of some genes present in one chromosome
Tandemly Repeated Gene Sequences (VTNR’s)
Gene amplification
Cells produce multiple copies of a gene by selective replication

24. Differential mRNA Processing (Posttranscriptional Control)
Cells in each tissue produce own version of mRNA
Same gene can be used to produce a certain protein in one tissue and a related, but slightly different protein in another tissue
Example: troponin
a protein that regulates muscle contraction
produced in different muscle tissues

25. Other Methods of Posttranscriptional Control
Proteolytic Protein Processing
Proteins produced in inactive form
Become active via removal of a portion of their polyepeptide chain
Chemical Modification
Addition or removal of functional groups
Affects enzyme activity
Kinases (add phosphate groups)
Phosphatases (remove phosphate groups)