Gene regulation in prokaryotes occurs through operons, clusters of genes that are expressed together. The lac and trp operons are examples of inducible and repressible operons, respectively. The lac operon is induced by the presence of lactose and involves both positive and negative control. It is negatively controlled by the lac repressor binding to the operator, but activated by the catabolite activator protein. The trp operon is repressed by the binding of tryptophan to the trp repressor, preventing transcription. It also uses attenuation, where mRNA folding terminates transcription at high tryptophan levels. Operons regulate metabolic pathways through induction or repression in response to environmental conditions.

1. Gene Regulation
in Prokaryotes.
'Operon Concept'
By
Mrs. Ratnamala Sarma

2. Introduction
• The process of “turning on” a gene to produce
mRNA and protein is called gene expression.
• The regulation of gene expression conserves energy
and space.
• Each cell controls, when its genes are expressed,
how much of the protein is made, and when it is
time to stop making that protein because it is no
longer needed.

3. Prokaryotic cells

5. Basic levels in gene regulation

6. Prokaryotic gene expression
• Operon Concept
Proteins that are needed for a specific function, or that
are involved in the same biochemical pathway, are often
encoded together in blocks called operons.
(Cluster of genes working and expressing together.)
• Lac operon, Ara operon, Gal operon, Trp
operon........etc.... 200 types of operon.
• These (Lac, Ara, Gal etc) are metabolites.
• Most preferred metabolite of bacteria is -'glucose '.

8. Organization of operon
Each operon needs only one
Regulatory region, including a
promoter, where RNA polymerase
binds, an operator, where other
regulatory proteins bind and an
Enhancer, where an activator binds.
Finally a transcription unit.

9. Regulatory gene

11. Positive control
&
Negative control

12. Lac operon

13. Lac operon
• It is a catabolic and Inducible operon
• Lac - lactose, a disaccharide (glucose and galactose
linked together by 4 glycosidic linkage.
• Lac operon is under control of two regulatory genes.
1. Activator called CAP (catabolite activator protein)
2. Lac Repressor (encoded by lac I gene).
Lac I located near the other lac genes, but transcribe by its
own promoter.

15. The 3 structural genes code for enzymes.
Lacz - breaks lactose iinto glucose and galactose.
Lac y- transport of lactose to tthe Cell.
LacA - frees cell from toxic TONPG ( a toxin).

16. Mechanism of Lac operon regulation.
 Negative control of lac operon
 lac repressor itself has its own promoter which constitutively directs the expression of
Lacl gene. Thus, lac repressor is always produced in the cell. But the repressor can bind
to the operator only in absence of lactose.
• Lac operator is the DNA sequence near the promoter and a portion of which overlaps
with the polymerase binding site within the promoter. If operator is free, polymerase
can bind over promoter; if operator is engaged by repressor, polymerase cannot bind at
the promoter.
• If lactose absent in the medium then, Lac repressor would be free and active: will bind
to operator. RNA polymerase cannot bind to the promoter.
• Presence of lactose, allolactose binds with the repressor inducing a conformational
change which renders lac repressor inactive.
• When lac repressor is inactive, it cannot bind with the operator. When operator is free,
RNA polymerase can bind at the promoter and can carry out transcription of
downstream lac operon genes.
• This is the negative control of lac operon which involves repressor protein.

17. Positive control of Lactose operon
Involving the activator protein. CAP
• When glucose is absent, CAP binds at a site called CAP
binding site, which alters expression in positive way.
• Thus the combined effect of these two regulators
ensures that the genes are expressed at significant levels
only when lactose is present and glucose absent

18. Trp operon
• The Trp operon Anabolic and Repressible operon.
• Charles Yanofsky and his co-workers explicitly studied the
role of regulatory and structural genes of the trp operon.
• Only one codon for Tryptophan in genetic codon.
(Can be off if created in excess)
• Controlled at the levels of premature transcription
termination.
• The trp operon has five continuous trp genes that encode
enzymes for biosynthesis of amino acid tryptophan.

20. Mechanism
• Two mechanisms regulate the trp operon.
1. Repressor or derepression mechanism
2. Attenuation mechanism
• Tryptophan acts as co-repressor, as binds with the
repressor protein and both binds to operator not
allowing polymerase to join the promoter and lowers
the synthesis of its own genes.
• When tryptophan is absent in the medium or present
in limiting concentrations it cannot bind with repressor
and operator and promoter are free for polymerase to
bind synthesize trp mRNA.

21. High level of tryptophan
• TrpR gene of the tryptophan operon releases apo-repressor (inactive) protein
that alone cannot attach to the operator region. In the presence of a
corepressor or tryptophan, the apo-repressor protein activates and blocks the
RNA polymerase.
• Thus the respective RNA polymerase can neither bind with the operator gene
nor transcribe structural genes at a high tryptophan concentration. Expression
of trp operon during availability of tryptophan indicates that the operon system
will switch off to terminate the transcription.
•

22. Low level of Tryptophan
1. Lack of sufficient effector
molecules, the active
repressor protein attached
to the operator region will
detach. After its dissociation,
the repressor remains
inactive and functionless.
2. RNA polymerase becomes
free to transcribe structural
genes further to produce
tryptophan.
3. The expression of trp operon
during unavailability of
tryptophan means that the
operon system will switch on
to conduct transcription of
structural genes by the RNA
polymerase.

23. Attenuation
• It is the second regulatory region of the trp operon controlled by the trpL
gene or attenuator.
• Once the bacterial DNA is transcribed into mRNA, the attenuator sequence can
form dimers due to the pairing of palindromic sequences.
• ( There are four domains in the leader sequence, in which domain-3 can pair
with either domain-2 or domain-4, and domain-1 can pair with domain-2.
Besides, it comprises two trp residues.)
• The pairing of domain-2 and 3 results in antitermination.Pairing of domain-3
and 4 causes a termination of trp biosynthesis.
• The presence of domain-4 (also called attenuator) is important to terminate the
transcription because it only can facilitate stem-loop formation.

24. At low
tryptophan level
1. The ribosome sits at domain-
1 of the mRNA transcript.
Then, it translates the mRNA
very slowly due to the low
tryptophan level. As a result,
domain-3 interacts with
domain-2 due to the halt of
the ribosome at domain-1. In
such a case, the stem and
loop structure will not form.
As a result, the transcription
may continue to synthesize
enzymes necessary for trp
production.

25. At high
tryptophan level
1. The ribosome rapidly translates
the domain-1 and sits at the
domain-2 when the
concentration of tryptophan is
high inside the cell. As a result,
domain-3 associates with
domain-4 and aids in forming a
hair-loop structure
2. The dimerization of domain-3
and 4 cause the RNAp to fall off
and prevent mRNA from
transcribing genes encoding
enzymes for the trp biosynthesis.
Therefore, an attenuator
functions as a barrier at high trp
concentration due to the pairing
of self-complementary
sequences.

26. Thank you