1. CHAPTER 16 LECTURE SLIDES
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2. Control of Gene Expression
Chapter 16

3. Control of Gene Expression
Controlling gene expression is often accomplished by controlling transcription initiation
Regulatory proteins bind to DNA
May block or stimulate transcription
Prokaryotic organisms regulate gene expression in response to their environment
Eukaryotic cells regulate gene expression to maintain homeostasis in the organism

4. Prokaryotic regulation
Control of transcription initiation
Positive control – increases frequency of initiation of transcription
Activators enhance binding of RNA polymerase to promoter
Effector molecules can enhance or decrease
Negative control – decreases frequency
Repressors bind to operators in DNA
Allosterically regulated
Respond to effector molecules – enhance or abolish binding to DNA

5. Prokaryotic cells often respond to their environment by changes in gene expression
Genes involved in the same metabolic pathway are organized in operons
Induction – enzymes for a certain pathway are produced in response to a substrate
Repression – capable of making an enzyme but does not

6. lac operon Contains genes for the use of lactose as an energy source
b-galactosidase (lacZ), permease (lacY), and transacetylase (lacA)
Gene for the lac repressor (lacI) is linked to the rest of the lac operon

8. The lac operon is negatively regulated by a repressor protein
lac repressor binds to the operator to block transcription
In the presence of lactose, an inducer molecule (allolactose) binds to the repressor protein
Repressor can no longer bind to operator
Transcription proceeds
Even in the absence of lactose, the lac operon is expressed at a very low level

9. a. Glucose Low , Inducer Present, Promoter Activated DNA Allolactose
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Glucose Low , Inducer Present, Promoter Activated
DNA
Allolactose
Repressor will
not bind to DNA
cAMP–CAP binds
to DNA
CAP-
binding
site
mRNA
Glucose level
is low cAMP
is high A
CAP Y
cAMP
cAMP Z
cAMP activates
CAP by causing a
conformation change RN A
polymerase is not blocked
and transcription can occur a.

10. blocked by the lac repressor
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Glucose High, Inducer Absent, Promoter Not Activated
Glucose is available
cAMP level is low
Repressor binds to DNA A
CAP does
not bind Y
Effector site is empty,
and there is no
conformation change
RNA polymerase is
blocked by the lac repressor b.

11. Glucose repression
Preferential use of glucose in the presence of other sugars
Mechanism involves activator protein that stimulates transcription
Catabolic activator protein (CAP) is an allosteric protein with cAMP as effector
Level of cAMP in cells is reduced in the presence of glucose so that no stimulation of transcription from CAP-responsive operons takes place
Inducer exclusion – presence of glucose inhibits the transport of lactose into the cell

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13. trp operon Genes for the biosynthesis of tryptophan
The operon is not expressed when the cell contains sufficient amounts of tryptophan
The operon is expressed when levels of tryptophan are low

14. The trp operon is negatively regulated by the trp repressor protein
trp repressor binds to the operator to block transcription
Binding of repressor to the operator requires a corepressor which is tryptophan
Low levels of tryptophan prevent the repressor from binding to the operator

17. Eukaryotic Regulation
Control of transcription more complex
Major differences from prokaryotes
Eukaryotes have DNA organized into chromatin
Complicates protein-DNA interaction
Eukaryotic transcription occurs in nucleus
Amount of DNA involved in regulating eukaryotic genes much larger

18. Transcription factors
General transcription factors
Necessary for the assembly of a transcription apparatus and recruitment of RNA polymerase II to a promoter
TFIID recognizes TATA box sequences
Specific transcription factors
Increase the level of transcription in certain cell types or in response to signals

20. Promoters form the binding sites for general transcription factors
Mediate the binding of RNA polymerase II to the promoter
Enhancers are the binding site of the specific transcription factors
DNA bends to form loop to position enhancer closer to promoter

22. Eukaryotic chromatin structure
Structure is directly related to the control of gene expression
DNA wound around histone proteins to form nucleosomes
Nucleosomes may block access to promoter
Histones can be modified to result in greater condensation

23. Methylation once thought to play a major role in gene regulation
Many inactive mammalian genes are methylated
Lesser role in blocking accidental transcription of genes turned off
Histones can be modified
Correlated with active versus inactive regions of chromatin
Can be methylated – found in inactive regions
Can be acetylated – found in active regions