2. Discovery of the Operon
In 1961, Jacob and Monod studied the metabolism of lactose by E.
coli
Three enzyme activities / three genes were induced together by
galactosides
Based on their observation they propossed operon concept
Nobel prize in the year 1965
3. Introduction
An operon is a collection of prokaryotic genes
transcribed together on a single mRNA transcript
(polycistronic) to serve a single purpose
Composed of
An operator, an “on-off” switch
A promoter
Genes for metabolic enzymes
Can be switched off by a repressor protein
A corepressor is a small molecule that binds to a
repressor to switch an operon off
5. Operons are either REPRESSIBLE or
INDUCIBLE
REPRESSIBLE: INDUCIBLE:
Involves binding a
co-repressor
(metabolite/end-
product) to
repress/turn OFF
operon transcription
Involves binding an
inducer
(metabolite/starting
substrate) to
induce/turn ON
operon transcription
6. 3 combinations found in nature:
REPRESSIBLE INDUCIBLE
NEGATIVE
This operon uses a
repressor that binds a co-
repressor to repress
transcription.
This operon uses a
repressor that binds an
inducer to induce
transcription.
POSITIVE Not found.
This operon uses an
activator that binds an
inducer to induce
transcriptionn.
7. Lac operon
The lac operon was the first operon discovered
It contains 3 genes coding for E. coli proteins that permit
the bacteria to use the sugar lactose
LacZ encodes Beta-galactosidase
breaks up lactose into glucose and galactose (galactose also converted to
glucose for metabolism)
Isomerizes lactose into allolactose inducer (presence of lactose means
presence of allolactose)
LacY encodes permease
For lactose transport across cell membrane
LacA encodes transacetylase
Poorly understood function
8. The Lac operon has 2 control circuits:
NEGATIVE INDUCIBLE
Uses a repressor that binds an inducer
(allolactose) to induce operon transcription.
Requires presence of lactose
POSITIVE INDUCIBLE
Uses an activator (CAP) that binds an inducer
(cyclic AMP) to induce operon transcription.
Requires absence of (preferred) glucose
9. LAC OPERON NEGATIVE INDUCIBLE CONTROL CIRCUIT:
• In the absence of lactose, an active repressor protein
binds to the operator and blocks transcription by RNA
Polymerase:
10. •When lactose is present in the cell, allolactose, an
isomer of lactose, binds to the repressor.
•This inactivates the repressor, because it can no
longer bind the operator.
•Now RNA Polymerase can transcribe the Lac operon:
11. RNA polymerase have low affinity
towards lac operon
So even the operon is on
transcription will occur poorly
There comes the role of positive
inducible control
12. LAC OPERON POSITIVE INDUCIBLE CONTROL
CIRCUIT
Attachment of RNA polymerase to the promoter requires the
presence of a Catabolite gene Activatior Protein (CAP) that is
bound to the cAMP
Presence of glucose lowers the cAMP by inactivating adenylyl
cyclase responsible for the synthesis of cAMP
Due to lesser cAMP formation of CAP-cAMP is low
So RNA polymerase attachment and transcription will be
negligible in the presence of glucose
As CAP-cAMP is essential for transcription of lac operon it is
considered as positive regulator.
16. The Trp operon has 2 control mechanisms
NEGATIVE REPRESSIBLE OPERON
Uses a repressor that binds a co-repressor (end
product Trp) to repress operon transcription by 70-
fold
Requires presence of Trp
ATTENUATION
Involves premature transcription termination
Requires high Trp levels
17. TRP OPERON NEGATIVE REPRESSIBLE CONTROL CIRCUIT: By itself, the operon
is on. RNA polymerase can bind to the promotor and moves freely through the
operator to transcribe the genes:
18. When co-repressor (end-product) Trp is present, it binds
to the repressor. This activates the repressor, causing it
to bind the operator to block Trp operon transcription:
