Chapter 18 - Operons
Control of Gene Expression in
Prokaryotes

Sample Pathway for Control of
Tryptophan (trp) Expression
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
transcription
translation
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

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

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/conte
nt/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

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!

operons, using proteins that inhibit or
promote binding of RNA pol.
–
Distal and Proximal Control Elements
–
Proteins involved include:
•
Transcription factors
•
Activators
•
Mediator ProteinsPost-Transcriptional Regulation
RNA Processing
– differential/alternative splicing can produce
different 2
0
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