Post on 30-Dec-2015
transcript
Chapter 13. Regulation of gene expression
References: 1. Stryer: “Biochemistry”, 5th Ed.2. Hames & Hooper: “Instant Notes
in Biochemistry”, 2nd Ed.
1. Basic principles of gene control
What is gene expression?
It is a process by which a gene's information is converted into the structures and functions of a cell.
For a protein encoding gene, it is a process of the transcription of a gene into mRNA, and the latter is translated into protein.
How to control gene expression?
In prokaryotic cells, gene expression is regulated primarily at the level of transcription by metabolites;
In eukaryotes it is controlled by much more complex mechanisms, including the action of protein factors, hormones, etc.
Control of gene expression in eukaryotes
DNA
Protein
mRNARNA
synthesis
InactivemRNA
mRNA
InactiveProtein
Transcript.control
1
RNA processingcontrol
2
RNAtransportcontrol
3
mRNA degradation
control
5
Translationcontrol
4
Protein activitycontrol6
Nucleus
Cytosol
2. Regulation of Prokaryotic gene expression– the operon theory
Operons are groups of genes that function to produce proteins needed by the cell.
There are two types of genes in operonsA. Structural genes code for proteins
needed for the normal operation of the cell.
B. Regulator genes code for proteins that regulate other genes.
lac I lac Z lac A lac Y PlacI Plac Olac
Plac : promoter; Olac: operator; lac Z: -galactosidase; lacY: permease; Lac A: transacetylase; lac I: structural gene for lac repressor; PlacI : promoter for lac I.
Lactose Operon
Regulation of the lac operon in E. coli.
p i p o z y a
The lact Operon
RepressormRNA
Repressor
The repressor binds to the operator region and prevents RNA polymerase from transcribing the structural genes
In the presence of lactose
p i p o z y a
The lact Operon
Lactose binds the repressor and allows RNA polymerase access to the operator, resulting in transcription of the structural gene
-galactosidase permease transacetylase
RepressormRNA
Repressor
Lactose
mRNA
In the presence of glucose & lactose
p i p o z y a
The lact Operon
glucose
AdenylcyclasecAMP ATP
L actose
Low level of transcription
In the absence of glucose and presence of lactose
p i p o z y a
The lact Operon
L actose
High level of transcription
AdenylcyclasecAMP ATP
CAP
active
CAP: catabolite activator protein
Tryptophan Operon
The trp operon encodes the genes for the synthesis of tryptophan
L A BP,O a CDE
Genes for enzymes
P: promoter, O: operator, a: attenuator, L: leader sequence, A,B,C,D,E: enzymes
of the Trp pathway
Regulation of the trp operon in E. coli.
Binding of Trp to the trp repressor increases the activity of the repressor, and therefore Trp is known as a co-repressor.
Expression of the trp operon is also regulated by attenuation. The attenuator region contains tandem Trp codons, which play a role to attenuate the transcription of the following genes.
1) The regulation of eukaryotic gene expression differs from that of prokaryotes in:
A) Eukaryotic RNA polymerases cannot transcribe DNA on their own—a multisubunit transcriptional apparatus must assemble first at the TATA box of the gene.
3. Regulation of eukaryotic gene expression.
B) Activators and repressors of eukaryotic gene expression act by altering the rate of formation of the transcriptional complex, while those of prokaryotes act by binding to the promoter or operator.
C) Eukaryotic genes are controlled by multiple proteins rather than by just one or two.
2) Zinc fingers regulate eukaryotic gene expression by binding to DNA strands
C
HC
H
HC
HCZ n Z n
Z in c f in g e r L in k e r Z in c f in g e r Zinc finger Linker Zinc finger
For example, TFIIIA is a transcriptional factor for transcription of the 5S genes by RNA polymerase III. It contains nine Zinc fingers. The Zinc fingers bind to DNA in the major groove to form the initiation complex.
C
HC
H
HC
HCZn Zn
DNA
Steroid hormones (e.g. cortisol, testosterone, progesterone) play their roles by forming hormone-receptor complexes, which then bind to specific sites on DNA, and induce or repress the transcription of the particular gene.
3) Regulation of eukaryotic gene expression by hormones
They are eukaryotic transcriptional regulators, which form dimers to bind DNA strands around the major groove.
Leucine zipper proteins mediate the effect of cAMP on transcription by binding to the cAMP response element (CRE).
4) Leucine zipper proteins:
CRE structure is a palindromic 8-bp DNA sequence:
5’-TGACGTCA-3’
3’-ACTGCAGT-5’
5’-TGACGTCA-3’
3’-ACTGCAGT-5’
CRE binding protein
For example, cAMP response element binding protein (CREB) has the leucine zipper region:
cAMP protein kinase A (PKA) phosphorylation of CREB dimerization of CREB binding of CREB to DNA activation of transcription of the gene.
Signaling through cAMP & PKA to CREB
PP
Phosphorylation
Dissociation Nuclear translocation
Target gene activation
Cytoplasm
Nucleus
AC
cAMP
C
C
C
C
CBP
P
PKA
CREB
Dimerization of a zipper protein increases its efficacy as a transcriptional activator.
CREB is a target for the Ca2+-calmodulin-activated CaM kinase II, protein kinase C, and other kinase, which suggests that it integrates many signals for DNA transcription.