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Chapter 18 - Operons
Control of Gene Expression inProkaryotes
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Sample Pathway for Control of
Tryptophan (trp) Expression
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
transcript
ion
translatio
n
Enzyme 1
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
Each enzyme
catalyzes the
next series ofreactions
necessary for
tryptophan
production
Promotor (RNA
polymerase binding
site)
DNAmRNA
(transcription unit)2 Negative Feedback
shut-off systems
trp
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Basic Vocabulary
mRNA Transcription Unit:
- grouping of mRNA genes with related
function
- 2 advantages ~
1. organization
2. easy regulationone on/off switch can be used to control an
entire cluster
of related genes
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Basic Vocabulary
Operon:
- an entire gene-regulating system in
PROKARYOTES, which includes the
operator, promotor, and all corresponding
genes
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Basic Vocabulary
Operator:
- on-off switch to genes
- controls the access of RNA polymeraseto genes
- found within the promotor region or
between the promoter and gene-encodingregions
Lets take a closer
look at the tryptophan
(trp) operonanexample of a
repressor system
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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 poly
merase
Trp operon controls the production
of the amino acid tryptophan
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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 trpre ressor
Inactive trp
repressor
protein
trp
trp
trp
trp trp
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How is the trp operon turned off
once enough trp is made?
Active trp repressorcan now bind to
operator
RNA polymerse,
therefore is
physically blocked
from transcribing
genes for trp
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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.
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Watch this animation to fullyunderstand and review this
operon,
then take the on-line quiz together
as a class
http://bcs.whfreeman.com/thelifewire/content
/chp13/1302002.html
http://bcs.whfreeman.com/thelifewire/content/chp13/1302002.htmlhttp://bcs.whfreeman.com/thelifewire/content/chp13/1302002.htmlhttp://bcs.whfreeman.com/thelifewire/content/chp13/1302002.htmlhttp://bcs.whfreeman.com/thelifewire/content/chp13/1302002.html
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Overview of Inducible Lac Operon
The Lac operon controls the production of
the -galactosidase, an enzyme that
catalyzes the hydrolysis (break-down) of
lactose into glucose and galactose.
This is an inducible operon, meaning gene
expression -galactosidase is stimulated
by the presence of an co-inducer, lactose.
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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 -
galactosidase
production
mRNA for
inducerprotein
inducer
protein in
active
form
RNA pol.
blocked from -
galactosidase
transcription
without lactose
co-inducer
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TWO regulatory mechanisms used
to turn on lac operon
1) Presence of lactose as a co-inducer
** reason?
-galactosidase is not needed unless lactose
needs to be broken down
2) Low amounts of glucose
** reason?
Recall that lactose breaks down into glucose andgalactose. 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.
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TWO regulatory mechanisms used
to turn on lac operon
1) Allolactose co-
inducer attaches to
allosteric site of induc
protein, inactivating it
AlloLactose
2. cAMP attaches to CRP
transcription factor when
glucose is scarce
CRP Transcription
Factor
cAMP
RNA polymerase can bindonly with the help of CRP
transcription factor
RNA pol.
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Lac Operon Animation
Watch this animation to fully understand and
review the lac operon!
http://highered.mcgraw-
hill.com/olc/dl/120080/bio27.swf
http://highered.mcgraw-hill.com/olc/dl/120080/bio27.swfhttp://highered.mcgraw-hill.com/olc/dl/120080/bio27.swfhttp://highered.mcgraw-hill.com/olc/dl/120080/bio27.swfhttp://highered.mcgraw-hill.com/olc/dl/120080/bio27.swfhttp://highered.mcgraw-hill.com/olc/dl/120080/bio27.swf
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How are genes controlled in
eukaryotes?
1) Regulation of Chromatin Structure
2) Pre and post Transcriptional Regulation
3) Pre and Post Translational Regulation
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Regulation of Chromatin Structure
N-terminus (amino group) of histone proteins face outwards
from nucleosome
Tails are thus able to be modified chemically
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Regulation of Chromatin Structure
Histone Acetylationneutralizes (+) charges on
tails, which prevents binding to adjacentnucleosomeloose
chromatin structure
results, allowing for increased transcription
MethylationPromotes condensation
Phosphorylationcan prevent condensation, ifphosphorylation is
adjacent to methyl group
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Pre-Transcriptional Regulation
Similar to methods used in bacterialoperons, using proteins that
inhibit or
promote binding of RNA pol.
Distal and Proximal Control Elements Proteins involved
include:
Transcription factors
Activators
Mediator Proteins
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Post-Transcriptional Regulation
Time of mRNA degradation can vary
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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
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Post-Translational Regulation
During protein processing, folding
Timing of protein degredation can vary
Proteasomes degrade proteins that aretagged by ubiquitine
molecules
