chapter 15 – gene expression
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Chapter 15 – Gene Expression. Control of Gene Expression in Prokaryotes and Eukaryotes. trp. Sample Pathway for Control of Tryptophan (trp) Expression. Promotor (RNA polymerase binding site). transcription. translation. Enzyme 1. Gene 1. - PowerPoint PPT PresentationTRANSCRIPT
Chapter 15 – Gene Expression
Control of Gene Expression in Prokaryotes and Eukaryotes
Sample Pathway for Control of Tryptophan (trp) Expression
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
tran
scrip
tion
tran
slat
ion
Enzyme 1
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
Each enzyme catalyzes the next series of reactions necessary for tryptophan production
Promotor (RNA polymerase binding site)
DNA mRNA (transcription unit)
2 Negative Feedbackshut-off systems
trp
Basic Vocabulary
mRNA Transcription Unit:
- grouping of mRNA genes with related function
- 2 advantages ~
1. organization
2. easy regulation – one “on/off” switch can be used to control an entire cluster of related genes
Basic Vocabulary
Operon:
- an entire gene-regulating system in PROKARYOTES, which includes the operator, promotor, and all corresponding genes
Basic Vocabulary
Operator:
- “on-off” switch to genes
- controls the access of RNA polymerase to genes
- found within the promotor region or between the promoter and gene-encoding regions
Let’s take a closer look at the tryptophan (trp)
operon…an example of a “repressor” system
Overview of Repressor Trp Operon Components
r. Trp Repressor Gene (always “on”)
promotor
rp. RNA Pol. binding site
o. Operator
r. rp. o. Gene 1
5 Genes needed for tryptophan production
mRNA transcription unit
Enz 1 Enz 2 Enz 3 Enz 4 Enz 5
Gene 1Gene 3 Gene 4 Gene 5
trp
mRNA for trp repressor
Trp repressor protein translated in its “inactive” form
RNA polymerase
Trp operon controls the production of the amino acid tryptophan
How is the trp operon turned off once enough trp is made?
Trp repressor gene
RNA pol binding site
operator
Increased concentration of trp (co-repressor) increases chances of trp binding to allosteric site of inactive trp repressor
Inactive trp repressor protein
trp
trp
trp
trp trp
How is the trp operon turned off once enough trp is made?
Active trp repressor can now bind to operator
RNA polymerse, therefore is physically blocked from transcribing genes for trp
Why is this a repressor system?
The trp operon is a repressor operon, meaning gene expression of the operon is repressed by the presence of the co-repressor, tryptophan.
Watch this animation to fully understand and review this operon, then take the on-line quiz together
as a class
http://bcs.whfreeman.com/thelifewire/content/chp13/1302002.html
Overview of Inducible Lac Operon
The Lac operon controls the production of the ß-galactosidase, an enzyme that catalyzes the hydrolysis (break-down) of lactose into glucose and galactose.
This is an inducible operon, meaning gene expression ß-galactosidase is stimulated by the presence of an co-inducer, lactose.
Inducible Lac operon
I. crp. rp. o.
I. Lac Inducer
crp: helps RNA pol. to bind
rp: RNA pol.binding site
o. operator
promotorGenes that promote ß-galactosidase production
mRNA for inducer protein
inducer protein in “active” form
RNA pol. blocked from ß-galactosidase transcription without lactose co-inducer
TWO regulatory mechanisms used to turn on lac operon
1) Presence of lactose as a co-inducer
** reason?
ß-galactosidase is not needed unless lactose needs to be broken down
2) Low amounts of glucose
** reason?
Recall that lactose breaks down into glucose and galactose. Low glucose levels signals the cell for more lactose to be broken down.
Both conditions must be met for the lac operon to turn on.
TWO regulatory mechanisms used to turn on lac operon
1) Allolactose co-inducer attaches to allosteric site of inducer protein, inactivating it
AlloLactose
2. cAMP attaches to CRP transcription factor when glucose is scarce
CRP Transcription Factor
cAMP
RNA polymerase can bind only with the help of CRP transcription factor
RNA pol.
Lac Operon Animation
Watch this animation to fully understand and review the lac operon!
http://highered.mcgraw-hill.com/olc/dl/120080/bio27.swf
How are genes controlled in eukaryotes?
1) Regulation of Chromatin Structure
2) Pre and post Transcriptional Regulation
3) Pre and Post Translational Regulation
Regulation of Chromatin Structure
N-terminus (amino group) of histone proteins face outwards from nucleosome
Tails are thus able to be modified chemically
Regulation of Chromatin Structure
• Histone Acetylation – neutralizes (+) charges on tails, which prevents binding to adjacent nucleosome loose chromatin structure results, allowing for increased transcription
• Methylation Promotes condensation
• Phosphorylation can prevent condensation, if phosphorylation is adjacent to methyl group
Pre-Transcriptional Regulation• Similar to methods used in bacterial
operons, using proteins that inhibit or promote binding of RNA pol.– Distal and Proximal Control Elements– Proteins involved include:
• Transcription factors• Activators• Mediator Proteins
Post-Transcriptional Regulation
RNA Processing
– differential/alternative splicing can produce different 20 mRNA transcript
Differential splicing redefines which RNA segments are considered introns and which are exons
Post-Transcriptional Regulation
• Time of mRNA degradation can vary
Pre-Translational Regulation
Initiation of Translation
- can be blocked by regulatory proteins that prevent ribosome binding
- shortened polyA tails in mRNA prevents translation (polyA tails can be added during appropriate time)
- global regulatory control of all mRNAs in cell
Post-Translational Regulation
• During protein processing, folding
• Timing of protein degredation can vary
• Proteasomes degrade proteins that are tagged by ubiquitine molecules