1. Regulation of Gene Expression
Chapter 18
Regulation of Gene Expression

2. Overview: Gene Expression
Prokaryotes & eukaryotes use different methods to turn genes “on” or “off”
Ultimately gene expression is about efficient use of resources/energy
If you don’t need it, don’t make it
© 2011 Pearson Education, Inc.

3. Operons: The Basic Concept
Controls transcription
An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control
© 2011 Pearson Education, Inc.

4. Parts of the operon Promoter (RNA Polymerase binding site) Genes
Operator (Repressor binding site)
Genes

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

6. Other players:
Repressor: A protein that can turn the operon “off” or “on” by binding to the operator, blocking RNA polymerase
regulatory gene: makes the repressor (usually “upstream” from the operon.
Co-repressor: A molecule that binds to the repressor protein to change its shape
© 2011 Pearson Education, Inc.

7. Tryptophan (corepressor)
Figure 18.3b-1
DNA
mRNA
Protein
Active repressor
Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes.
Tryptophan (corepressor)
(b) Tryptophan present, repressor active, operon off

8. Figure 18.3
trp operon
Promoter
Promoter
Genes of operon
DNA
trpR
trpE
trpD
trpC
trpB
trpA
Operator
Regulatory gene
RNA polymerase
Start codon
Stop codon 3
mRNA 5
mRNA 5 E D C B A
Protein
Inactive repressor
Polypeptide subunits that make up enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
DNA
No RNA made
Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes.
mRNA
Protein
Active repressor
Tryptophan (corepressor)
(b) Tryptophan present, repressor active, operon off

9. Repressible and Inducible Operons: Negative or Positive Feedback?
Repressible operon: default is “on;” binding of a repressor shuts off transcription (ex: trp operon)
inducible operon default is “off”; a molecule called an inducer inactivates the repressor and turns on transcription (ex. lac operon)
© 2011 Pearson Education, Inc.

10. Figure 18.4
Regulatory gene
Promoter
Operator
DNA
DNA
lacI
lacZ
No RNA made 3
mRNA
RNA polymerase 5
Active repressor
Protein
(a) Lactose absent, repressor active, operon off
lac operon
DNA
lacI
lacZ
lacY
lacA
RNA polymerase
Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes. 3
mRNA
mRNA 5 5
Protein
-Galactosidase
Permease
Transacetylase
Allolactose (inducer)
Inactive repressor
(b) Lactose present, repressor inactive, operon on

11. When do we see inducible vs. repressible feedback?
Inducible enzymes usually in catabolic pathways;
Repressible enzymes usually function in anabolic pathways
© 2011 Pearson Education, Inc.

12. Eukaryotic gene expression is regulated at many stages
Chromatin modification
Transcription
RNA Processing
Transport to cytoplasm
Degradation of mRNA
Protein processing
Degradation of protein
Transport to cellular destination
© 2011 Pearson Education, Inc.

13. Figure 18.6
Signal
NUCLEUS
Chromatin
Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation
DNA
Gene available for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
RNA processing
Tail
Cap
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
Degradation of mRNA
Translation
Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells.
Polypeptide
Protein processing, such as cleavage and chemical modification
Active protein
Degradation of protein
Transport to cellular destination
Cellular function (such as enzymatic activity, structural support)

14. Differential Gene Expression
Cell differentiation results from gene expression,
Abnormalities in gene expression can lead to diseases including cancer
© 2011 Pearson Education, Inc.

15. Regulation of Chromatin Structure
Histone proteins, part of Chromatin are modified to allow for gene expression
1) Histone acetylation: acetyl groups are attached to positively charged lysines in histone tails
2) Phosphorylation: phosphate groups added next to methyl groups
This “loosens” the structure of the chromatin and allows for transcription.
(See video: DNA Packing)
© 2011 Pearson Education, Inc.

16. Amino acids available for chemical modification
Figure 18.7
Histone tails
DNA double helix
Amino acids available for chemical modification
Nucleosome (end view)
(a) Histone tails protrude outward from a nucleosome
Figure 18.7 A simple model of histone tails and the effect of histone acetylation.
Unacetylated histones
Acetylated histones
(b)
Acetylation of histone tails promotes loose chromatin structure that permits transcription

17. DNA Methylation- reduces transcription addition of methyl groups,
can cause long-term inactivation of genes in cellular differentiation
Phosphorylation counteracts methylation
© 2011 Pearson Education, Inc.

18. Epigenetic Inheritance
Although the chromatin modifications just discussed do not alter DNA sequence, they may be passed to future generations of cells
The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance
© 2011 Pearson Education, Inc.

19. Regulation of Transcription Initiation
Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery
© 2011 Pearson Education, Inc.