1. Differential Expression of Genes
Prokaryotes and eukaryotes precisely regulate gene expression in response to environmental conditions
In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types
RNA molecules play many roles in regulating gene expression in eukaryotes

2. Concept 18.1: Bacteria often respond to environmental change by regulating transcription
Natural selection has favored bacteria that produce only the products needed by that cell
A cell can regulate the production of enzymes by feedback inhibition or by gene regulation
One mechanism for control of gene expression in bacteria is the operon model

3. Operons: The Basic Concept
A cluster of functionally related genes can be coordinately controlled by a single “on-off switch”
The “switch” is a segment of DNA called an operator usually positioned within the promoter
An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control

4. The operon can be switched off by a protein repressor
The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase
The repressor is the product of a separate regulatory gene

5. The repressor can be in an active or inactive form, depending on the presence of other molecules
A corepressor is a molecule that cooperates with a repressor protein to switch an operon off
For example, E. coli can synthesize the amino acid tryptophan when it has insufficient tryptophan

6. By default the trp operon is on and the genes for tryptophan synthesis are transcribed
When tryptophan is present, it binds to the trp repressor protein, which turns the operon off
The repressor works only in the presence of its corepressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high

7. Polypeptide subunits that make up enzymes for tryptophan synthesis
Figure 18.3a
DNA
trp operon
Promoter
Promoter
Regulatory gene
Genes of operon
trpR
trpE
trpD
trpC
trpB
trpA
Operator
RNA polymerase
mRNA
Start codon 3′
mRNA 5′ 5′
Protein
Inactive repressor
E D C B A
Figure 18.3a The trp operon in E. coli: regulated synthesis of repressible enzymes (part 1: tryptophan absent)
Polypeptide subunits that make up enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on

8. Positive Gene Regulation
Some operons are also subject to positive control through a stimulatory protein, such as catabolite activator protein (CAP), an activator of transcription
When glucose (a preferred food source of E. coli) is scarce, CAP is activated by binding with cyclic AMP (cAMP)
Activated CAP attaches to the promoter of the lac operon and increases the affinity of RNA polymerase, thus accelerating transcription

9. When glucose levels increase, CAP detaches from the lac operon, and transcription returns to a normal rate
CAP helps regulate other operons that encode enzymes used in catabolic pathways

10. RNA polymerase binds and transcribes
Figure 18.5a
Promoter
Operator
DNA
lac I
lacZ
CAP-binding site
RNA polymerase binds and transcribes
Active
CAP
cAMP
Inactive lac repressor
Inactive
CAP
Figure 18.5a Positive control of the lac operon by catabolite activator protein (CAP) (part 1: glucose scarce)
Allolactose
(a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

11. RNA polymerase less likely to bind
Figure 18.5b
Promoter
DNA
lac I
lacZ
CAP-binding site
Operator
RNA polymerase less likely to bind
Inactive
CAP
Inactive lac repressor
Figure 18.5b Positive control of the lac operon by catabolite activator protein (CAP) (part 2: glucose present)
(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

12. Operons that can be turned on or off are inducible or repressable
Operons that are always on are called constitutive

13. On unless specifically turned off = negative control example = repressor action in the lac operon
Off unless specifically turned on = positive control example = CAP protein in lac operon

14. Chapter 18-Prokaryotic gene expression only
Section 18.1 Pp
Concept 18.1 p 378
Q1, Q6 on p 379