Lac operon concept

1. Regulation of gene
expression
Dr. N.R.Hazari

2.  Gene: A DNA segment that contains the
all genetic information required to
encodes RNA and protein molecules.
 Genome: A complete set of genes of a
given species.
 Gene expression: A process of gene
transcription and translation.

3.  Expression of many genes is relatively
continuous.
 Some genes are expressed only under
 certain conditions,
 nutritional
 during differentiation and development
 after physiological stimulations
(nervous,Hormonal etc)
 stressful situations.
 Thus there is mechanism by which the
cells express or turn off certain genes.

4.  High cost of protein synthesis, regulation
of gene expression is essential to making
optimal use of energy.
 Regulation of gene expression is
absolutely essential for the growth,
development, differentiation and very
existence of an organism.

5.  There are two types of gene regulation

 1. Positive Regulation
 2. Negative regulation.

6. Positive regulation
 The expression of gene is
increased by the presence of
specific regulatory element ,is said
positive regulation.
 The molecule which exert positive
regulation is said to be positive
regulator or activator / inducer
 The process is called Derepression
or Induction

7. Negative regulation
 The expression of gene is
decreased by the presence of
specific regulatory element ,is
said negative regulation.
 Molecule which exert the negative
regulation is said to be a negative
regulator or repressor.
 The process is called Repression.

8. Sites
 Gene regulation occurs at the
transcriptional, post-transcriptional,
translational and post-translational level.
 Controlling gene expression is often
accomplished by controlling transcription
initiation in prokaryotes.

9.  The environmental and metabolic state of
the cell has a direct and significant effect on
the control of gene expression.
 Usually small extracellular or intracellular
metabolites trigger the complex mechanisms.
 (stimulate or inhibit).
 All genes are not expressed at all time or in
all tissues.
 Ex: Insulin gene expressed only in
pancreatic cells but not in other tissues. That
means insulin gene is in repression state in
other tissue.

10. Type of gene expression
 Genes or gene expression are
considered under two categories:
1. Constitutive genes or expression:
 Some genes are essential and
necessary for life, and therefore are
continuously expressed, such as those
enzymes involved in TCA cycle. These
genes are called housekeeping genes.


11.  2. Inducible gene or expression
Induction and repression
The expression levels of some
genes fluctuate in response to the
external signals.
Ex: Tryptophan pyrrolase induced
by tryptophan.

12. Prokaryotic gene regulation.
 Prokaryotes provide models for the study of
gene expression in mammalian cells.
 Some features of prokaryotes gene
expression are unique.
 In prokaryotes, genes involved in metabolic
pathway are often present in linear array
called as OPERON.
 ( mRNA are polycistronic)
 Multiple genes are present on single
transcript and a single promoter initiates the
transcription all genes.

13. OPERON
Francois Jacob & Monod in 1961 first
describe operon model in E-coli.
An operon is a group
of genes that are
transcribed at the
same time.
They usually control
an important
biochemical process.
They are found only in
prokaryotes.

14.  Their theory was based on observations of
lactose metabolism in E-Coli.
 When E-Coli cells grown in glucose
medium cells don’t contain β-galactosidase
enzyme.
 But when cells are transferred to medium
contain only lactose, then β-galactosidase
level increases.
 β-galactosidase hydrolyses the lactose into
galactose & glucose.

15. Operon
 The gene for this enzyme is
clustered with other two genes
which produce two enzymes which
are involve in lactose metabolism
with β-galactosidase, i.e.
galactoside permease and
thiogalactoside transacetylase.

16.  Genes product involve in the same
pathway and initiate the transcription of
these genes by single promoter and
regulate by a single gene called operons.
 Some operons induced when metabolic
pathway needed.
For prokaryotic systems:
Operon is composed of structural genes,
promoter, operator, and other regulatory
sequences.

17. AYZOPI
structural gene
permease
regulatory site
operator
promoter
CAP-binding site
regulatory gene
transacetylase
β galactosidase
Lac Operon

18. Metabolism of lactose

19. Adapting to the environment
 E. coli can use either glucose, which is a
monosaccharide, or lactose, which is a
disaccharide.
 However, lactose needs to be hydrolysed
(digested) first.
 So the bacterium prefers to use glucose when
it can present and its genes are constitutive.

20. Regulation of Lac -operon
 Four situations are possible
1. When glucose is present and lactose is absent
the E. coli does not produce β-galactosidase.
2. When glucose is present and lactose is present
the E. coli does not produce β-galactosidase.
3. When glucose is absent and lactose is absent
the E. coli does not produce β-galactosidase.
4. When glucose is absent and lactose is present
the E. coli does produce β-galactosidase

21. Regulation of lac-operon : glucose is
present and lactose is absent
 IN the absences of lactose, the cell has
no need to produce lactose metabolizing
enzymes.
 Hence lac repressor protein , prevents
expression of genes of lactose
metabolism.

22. AYZOPI
mRNA
RNA pol
Situation I

23. Situation II
When lactose is present, lacZ, lacY, and lacA
genes are expressed.
AYZOPI
repressor
mRNA
lactoseallolactose
RNA pol
galactosidase

24. Situation III
AYZOPI RNA pol
When glucose is present, the [cAMP] is low, no CAP-cAMP is
formed and the expression of the lac operon is still low.

25. Situation 4
AYZOPI
CAP
cAMP
RNA pol
When glucose is absent and lactose is present, the CAP-
cAMP complex binds to the CAP site to activate the lac gene.

26. Carbohydrate
s
Activator
protein
Repressor
protein
RNA
polymeras
e
lac Operon
+ GLUCOSE
+ LACTOSE
Not bound
to DNA
Lifted off
operator site
Keeps falling
off promoter
site
No
transcription
+ GLUCOSE
- LACTOSE
Not bound
to DNA
Bound to
operator site
Blocked by
the repressor
No
transcription
- GLUCOSE
- LACTOSE
Bound to
DNA
Bound to
operator site
Blocked by
the repressor
No
transcription
- GLUCOSE
+ LACTOSE
Bound to
DNA
Lifted off
operator site
Sits on the
promoter
site
Transcription
Summary

27. THANK YOU!
RNA pol