18 regulation of gene expression - biologybiology-lifescience.com/ppt_109_files/chapter18...
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Control of Gene Expression
E. coli lac Operon
lacY DNA
Lac repressor
Regulatory gene
lacI
Binds Lac repressor
Binds RNA polymerase
Transcription initiation site
Promoter Operator
Sequences that control the expression of the operon
β-Galactosidase
lacZ
lac operon
Permease Transacetylase
Transcription termination site
lacA
Regulation of Inducible lac Operon
lacA DNA
mRNA Lac
repressor (active)
lacI
a. Lactose absent from medium
RNA polymerase cannot bind to promoter
Promoter Operator
Transcription blocked
When lactose is absent from the medium, the active Lac repressor binds to the operator of the lac operon, blocking transcription.
lac operon lacZ lacY
Inducible lac Operon
Transcription occurs
b. Lactose present in medium
DNA
Lac repressor
(active)
mRNA
Binding site for inducer
Allolactose (inducer)
RNA polymerase binds and transcribes operon
lacI Promoter Operator
When lactose is present in the medium, some of it is converted to the inducer allolactose. Allolactose binds to the Lac repressor, inactivating it so that it cannot bind to the operator. This allows RNA polymerase to bind to the promoter, and transcription of the lac operon occurs. Translation of the mRNA produces the three lactose metabolism enzymes.
Lactose metabolism enzymes
lacA
mRNA Translation
lacY
lac operon
lacZ
Inactive repressor
Repressable trp Operon
mRNA
DNA
a. Tryptophan absent from medium
Trp repressor (inactive)
mRNA
Regulatory gene
trpR
RNA polymerase binds and transcribes operon
Promoter Operator trpE
Transcription occurs
When tryptophan is absent from the medium, the Trp repressor is inactive in binding to the operator and transcription proceeds.
trp operon
trpD trpC trpB trpA
Tryptophan biosynthesis
enzymes
Translation
Regulation of the repressable trp Operon
trpE trpD trpC trpB trpA trpR Promoter Operator
trp operon
b. Tryptophan present in medium
Transcription blocked
DNA
mRNA
Trp repressor (inactive)
Tryptophan (corepressor)
RNA polymerase cannot bind to promoter
Trp repressor (active)
Tryptophan-binding site
When tryptophan is present in the medium, the amino acid binds to, and activates, the Trp repressor. The active repressor binds to the operator and blocks transcription.
Positive Regulation of lac Operon
DNA lacI
CAP site Promoter Operator
lacZ
a. Lactose present; glucose low or absent
mRNA
Lac repressor (active)
Allolactose (inducer)
Inactive repressor
cAMP
CAP
Active CAP
Transcription occurs
Translation
Lactose metabolism
enzymes
mRNA RNA polymerase
When lactose is present and glucose is low or absent, cAMP levels are high. cAMP binds to CAP, activating it. Active CAP binds to the CAP site and recruits RNA polymerase to the promoter. Transcription then occurs.
Positive Regulation of lac Operon
RNA polymerase binding site
b. Lactose present; glucose present
DNA
mRNA
Lac repressor (active)
Allolactose (inducer)
Inactive repressor
Inactive CAP
CAP site Promoter Operator
No transcription
RNA polymerase cannot bind
lacI lacZ
When lactose is present and glucose is present, cAMP levels are low. As a result, CAP is inactive and cannot bind to the CAP site. RNA polymerase then is unable to bind to the promoter, and no transcription occurs.
Gene Regulation Cytoplasm
Nucleus Chromatin
Pre-mRNA
DNA Determines which genes are translated
Transcriptional regulation • Chromatin remodeling to make genes accessible for transcription • Regulation of transcription initiation
Mature RNAs
Mature RNAs
Determines types and availability of mRNAs to ribosomes
Posttranscriptional regulation • Variations in pre-mRNA processing • Removal of masking proteins • Variations in rate of mRNA breakdown •RNA interference
Initiation of protein synthesis
Ribosome
Determines rate at which proteins are made
Translational regulation • Variations in rate of initiation of protein synthesis
New poly-peptide chains
Finished proteins
Protein breakdown
Determines availability of finished proteins
Posttranslational regulation • variations in rate of protein processing • Removal of masking segments • Varieties in rate of protein breakdown
Chromatin Remodeling Promoter
Promoter not accessible to proteins for transcription initiation
Promoter now accessible to proteins for transcription initiation Gene
Promoter
Remodeling complex
Chromatin remodeling exposes promoter
Activator
Gene
Regulatory sequence Nucleosomes
Organization of Eukaryotic Gene
DNA
Regulatory sequences
Enhancer
Promoter proximal elements
(regulatory sequences)
Promoter proximal
region
TATA box
Promoter
5' UTR
Transcription unit of gene
Exon Exon Exon Intron Intron
3' UTR
Transcription Complex on the Promoter
Transcription begins
Initial general transcription factor
Additional general transcription factors
Transcription complex
TATA box
Promoter
RNA polymerase
Site where transcription starts
DNA
DNA
The first general transcription factor recognizes and binds to the TATA box of a protein-coding gene’s promoter.
1
2 Additional general transcription factors and then RNA polymerase add to the complex, and then transcription begins.
Interactions Between Activators
Enhancer
Activators
Coactivator (multiprotein complex)
Interaction between activators at the enhancer, coactivator, and proteins at the promoter and promoter proximal region
Promoter Promoter proximal
region
Transcription initiation site
Gene
Maximal transcription
Gene
DNA loop
Activators
Combinatorial Gene Regulation
A unique combination of activators controls gene A.
DNA
Gene A, controlled by activators 2, 5, 7, and 8 binding to regulatory sequences in its enhancer
Gene A
Transcription Activators 2 5 7 8
2 5 7 8
Enhancer
Enhancer regulatory sequences
Combinatorial Gene Regulation
Gene B
Transcription Activators 1 5 8 11
Enhancer
Enhancer regulatory sequences
DNA 1 5 8 11
A different combination of activators controls gene B.
Gene B, controlled by activators 1, 5, 8, and 11 binding to regulatory sequences in its enhancer
Hormonal Gene Regulation
Hormone bound to receptor
Steroid hormone
Steroid hormone receptor
Protein
DNA
RNA polymerase
Transcription
Gene controlled by the steroid
hormone
Steroid hormone response element
Pre-mRNA
mRNA
mRNA + ribosomes
Gene Silencing via DNA Methylation
! The hemoglobin genes for instance are highly methylated and thus silenced in most vertebrate body cells except red blood cells.
! DNA methylation sometimes silences large blocks of genes or even whole chromosomes like one of the X chromosome in female mammals (Barr bodies)
! DNA Methylation underlies genomic imprinting in which either the paternal or maternal allele of a particular gene is silenced.
Genomic Imprinting: Prader-Willi and Angelman Syndrome
• Specific partial deletion of chromosome 15 results in:
Angelman syndrome if it’s from the mother
Prader-Willi syndrome if the chromosome is from the father
From chapter 13
Histone acetylation activates genes
The DNA around acetylated histones is less tightly wrapped around the histones in the nucleosome and thus more accessible to DNA binding proteins including transcription factors and RNA polymerase.
Gene Regulation Cytoplasm
Nucleus Chromatin
Pre-mRNA
DNA Determines which genes are translated
Transcriptional regulation • Chromatin remodeling to make genes accessible for transcription • Regulation of transcription initiation
Mature RNAs
Mature RNAs
Determines types and availability of mRNAs to ribosomes
Posttranscriptional regulation • Variations in pre-mRNA processing • Removal of masking proteins • Variations in rate of mRNA breakdown •RNA interference
Initiation of protein synthesis
Ribosome
Determines rate at which proteins are made
Translational regulation • Variations in rate of initiation of protein synthesis
New poly-peptide chains
Finished proteins
Protein breakdown
Determines availability of finished proteins
Posttranslational regulation • variations in rate of protein processing • Removal of masking segments • Varieties in rate of protein breakdown
RNAi
Nucleus
Cytoplasm
Other proteins bind and degrade one RNA strand
Dicer
Dicer Precursor RNA folds into stem-loop
miRNA gene
miRNA precursor
miRNA binds to complementary target mRNA
Degradation of mRNA
Inhibition of translation
Stepped Art
Double-stranded RNA
Protein Degradation
Development of Colorectal Cancer
Colon
Colon wall
Loss of tumor- suppressor gene APC (or other)
Normal colon epithelial cells
Small benign growth (polyp)
Larger benign growth (adenoma)
Activation of ras oncogene
Loss of tumor- suppressor gene DCC
Loss of tumor- suppressor gene p53
Additional mutations
Malignant tumor (carcinoma)