Regulation of gene expression... Lac operon

1. Regulation of gene expression..
By
Harinath Reddy

2. Regulation of gene expression
• Regulation of gene expression occur at various levels.
• Gene expression results in the formation of a
polypeptide…

3. In eukaryotes, the regulation could be exerted at
• (i) transcriptional level (formation of primary transcript).
• (ii) hnRNA processing level (regulation of splicing),
• (iii) transport of mRNA from nucleus to the cytoplasm.
• (iv) translational level.

4. • The genes in a cell are expressed to perform a particular
function or a set of functions.
In prokaryotes gene is regulate only at Transcription initiation:

5. • For example, if an enzyme called beta-galactosidase is
synthesised by E. coli.
• it is used to catalyse the hydrolysis of a disaccharide,
lactose into galactose and glucose;

6. • If the bacteria do not have lactose around them to be
utilised for energy source (Carbohydrate source).
• They would no longer require the synthesis of the
enzyme beta-galactosidase.

7. If E. coli in the presence of Lactose and absence of
glucose..

8. Gene expression regulated by
• Metabolic factors
• physiological factor
• environmental conditions
• The development and differentiation of embryo into
adult organisms are also a result of the coordinated
regulation of expression of several sets of genes..

9. In prokaryotes
• In prokaryotes, gene regulated at only transcription
initiation (At promoter region).
• The promoter regions of prokaryotic DNA is in many cases
regulated by the interaction of proteins (Repressor protein)
with operators.
• The operator bind a repressor protein.
P R P O Z Y A
RNA polymerase

10. Operator
• The promoter regions of prokaryotic DNA is in many
cases regulated by the interaction of repressor proteins
with sequences termed operators.
• The operator region is adjacent to the promoter
elements in most operons and in most cases the
sequences of the operator bind a repressor protein.

11. • Each operon has its specific operator and specific repressor.
• For example, lac operator is present only in the lac operon and it
interacts specifically with lac repressor only..

12. Lac operon in E.coli
• The elucidation of the lac operon was also a result of a
close association between
• Geneticist, Francois Jacob
• Biochemist, Jacque Monod

13. Lac operon
• In lac operon (here lac refers to lactose), a
polycistronic structural gene is regulated by a
common promoter and regulatory genes.
• Such arrangement is very common in bacteria
and is referred to as operon.

14. • To name few examples of operons..
• lac operon,
• trp operon,
• ara operon,
• his operon,
• val operon, etc.

15. • The lac operon consists of
• one regulatory gene (the i gene – inhibitor).
• three structural genes :
• Z gene:
• Y gene:
• A gene.

16. • Z gene: . The z gene codes for beta-galactosidase (β-
gal), which is primarily responsible for the hydrolysis
of the disaccharide.
• lactose hydrolysis into its monomers: galactose and
glucose.

17. • Y gene: codes for permease, which increases
permeability of the cell to β-galactosides..
• A gene: encodes a transacetylase.
• all the three gene products in lac operon are required
for metabolism of lactose.

18. Lac operon
(Catabolic process by Beta galactosidase enzyme)
• lac operon controls a catabolic pathway..

20. • The y gene codes for permease, which increases
permeability of the cell to β-galactosides (Lactose).
• The “a” gene encodes a transacetylase (Transfer acetyl
group to Beta galactosidase).
• Hence, all the three gene products in lac operon are
required for metabolism of lactose.

21. • Regulation of lac operon by repressor is referred to as
negative regulation.
• Lac operon is also under control of positive regulation as
well…

22. • Lactose is the substrate for the enzyme beta-
galactosidase and it regulates switching on and off of
the operon.
• Hence, it is termed as inducer…

23. • The lactose is transported into the cells through the
action of permease.
• Remember, a very low level of expression of lac operon
has to be present in the cell all the time..
• otherwise lactose cannot enter the cells.

26. • The repressor of the operon is synthesised (all-the-time
– constitutively) from the i gene.
• The repressor protein binds to the operator region of
the operon and prevents RNA polymerase from
transcribing the operon.

27. Dr. HarinathaReddy Aswartha
Assistant professor
Department of Microbiology
ANDHRAPRADESH
INDIA