Gene expression is the process by which information from a gene is used to produce a functional product like a protein. The document discusses gene regulation in E. coli using the lac operon as an example. The lac operon controls lactose metabolism and consists of 3 genes regulated by the lac repressor and catabolite activator protein (CAP). When glucose is present, the lac operon is off, but when glucose is low and lactose available, the operon is strongly expressed to use lactose.

1. PRESENTED BY:
MOUSAMI JARIA
ST. GEORGE COLLEGE OF MANAGEMENT AND SCIENCE
MSC MICROBIOLOGY
SEMESTER 2

2. It is the process by which information
from a gene is used in the synthesis of a
functional gene product.
When the information stored in our DNA
is converted into instructions for making
proteins or other molecules it is called
gene expression.

4. There are mainly two types of gene
regulation/ expression:
A. Positive regulation
B. Negative regulation.

5. When the expression of genetic
information is quantitatively increased
by the presence of specific regulatory
element , it is called as positive
regulation.
The element or molecule mediating
positive regulation is called positive
regulator.
Eg: E. coli ara operon

6. When the expression of genetic
information is quantitatively decreased
by the presence of specific regulatory
element, it is called as negative
regulation.
The element or molecule mediating
positive regulation is called negative
regulator.
Eg: E. coli lac operon.

7.  Lac operon is an operon or a group of genes
with a single promoter ( transcribed as single
mRNA). The genes in the operon encode
proteins that allow bacteria to use lactose an
energy source.
 It is an inducible catabolic operon of E. coli.
 It consists of:
1. Structural genes: It carries 3 structural
genes Z, Y & A.
 Codes respectively for : beta-galactosidse,
galactoside permease & thiogalactoside
transacetylase.

9.  E . coli can break down lactose , but if glucose
is available they use that.
 Glucose requires fewer steps and less energy to
break down than lactose.
 Howevr if lactose is the only sugar available,
E . coli uses it as energy source
 To use lactose, the bacteria must express the lac
operon genes, which encode key enzymes for
lactose uptake and metabolism . Only when 2
conditions are met:
• Lactose is available.
• Glucose is not available

10. Two regulator proteins are involved:
 The lac repressor, acts as lac sensor
 Catabolite activator protein (CAP), acts as a
glucose sensor
These proteins bind to the DNA of the lac
operon and regulate its transcription
based on lactose and glucose levels.

11.  The lac operon contains 3 genes : lacZ, lacY,
& lacA. These genes are transcribed as single
mRNA, under control of one promoter.
 lacZ encodes an enzyme called beta-
galactosidase which is responsible for
splitting lactose into readily usable glucose
and galactose.
 lacY gene encodes a membrane protein
called lactose permease , which is a
transmembrane ‘’pump’’ that allows the cell
to import lactose.

12.  The lacA gene encodes an enzyme known as
transacetylase that attaches a particular
chemical group to target molecules
 lac operon also contains a number of
regulatory DNA sequences. These are
regions of DNA to which particular
regulatory proteins can bind , controlling
transcription of operon.
 The promoter is the binding site for RNA
polymerase , the enzyme that performs
transcription.

13.  The operator is a negative regulatory site
bound by the lac repressor protein . The
operator overlaps with the promoter and
when the lac repressor is bound , RN
polymerase cannot bind to the promoter and
start transcription.
 The cap binding site is a positive regulatory
site that is bound by catabolite activator
protein(CAP). When CAP is bound to this
site , it promotes transcription by helping
RNA polymerase bind to the promoter.

14.  The lac repressor is a protein that represses
(inhibits) transcription of the lac operon. It
does this by binding to the operator, which
partially overlaps with the promoter. When
bound the lac repressor
gets in RNA polymerases
way and keeps it from
trancribing the operon.
.

15.  When lactose is not available , the lac
repressor binds tightly to the operator ,
preventing transcription by RNA polymerase
 But when lactose is present , the lac
repressor lose its ability to bind DNA. It
floats off the operator , clearing the way for
RNA polymerase to transcribe the operon.
 This change in the lac repressor is caused by
the small molecule allolactose ,an isomer of
lactose

16.  When lactose is available , some molecules
will be converted to allolactose inside the
cell.
 Allolactose binds to lac repressor and makes
it change shape so it can no longer bind
DNA.
 Allolactose is an example of an inducer , a
small molecule that triggers expression of a
gene operon.
 The lac operon is considered an inducible
operon because it is usually turned
off(repressed) , but can be turned on in the
presence of inducer allolactose.

17.  When lactose is present , the lac repressor loses its
DNA binding ability .
 This clears the way for RNA polymerase to bind to
the promoter and transcribe the lac operon
 RNA polymerase alone does not bind very well to
the lac operon promoter. It might makes a few
transcripts , but it won’t do much more unless it
gets help from catabolite activator protein(CAP)
 CAP binds to region of DNA just before the lac
operon promoter and helps RNA polymerase attach
to promoter, driving high levels of transcription.

18.  CAP is not always active (able to bind DNA),
instead its regulated by a small molecule
cyclic AMP(cAMP).
 cAMP is a ‘’ hunger signal’’ made by E. coli
when glucose levels are low.
 cAMP binds to CAP , changing its shape and
making it able to bind DNA and promote
transcription.
 Without cAMP , CAP cannot bind DNA and is
inactive.

19.  cAMP is only active when glucose levels are
low (cAMP levels are high).
 Thus the lac operon can only be transcribed
at high levels when glucose is absent.
 This strategy ensures that bacteria only turn
on the lac operon and start using lactose
after they have used up all of the preferred
energy source (glucose).

21. 1. GLUCOSE PRESENT LACTOSE ABSENT:
 No transcription of lac operon occurs
because lac repressor remains bound to
the operator and prevents transcription by
RNA polymerase
 Also cAMP levels are low because glucose
levels are high , so CAP is inactive and
cannot bind DNA

23. 2. GLUCOSE PRESENT LACTOSE PRESENT:
 Low level transcription of the lac operon
occurs
 The lac repressor is released from the
operator because the inducer (allolactose)
is present.
 cAMP levels are low because glucose is
present
 CAP remains inactive and cannot bind to
DNA , so transcription only occurs at a low
leaky level.

24. 3. GLUCOSE ABSENT , LACTOSE ABSENT:
 No transcription of lac operon occurs.
 cAMP levels are high because glucose levels
are low , so CAP is active and will bound to
DNA
 Lac repressor will also be bound to the
operator acting as a roadblock to RNA
polymerase and preventing transcription.

25. 4. GLUCOSE ABSENT, LACTOSE PRESENT:
 Strong transcription of the lac operon occurs
 The lac repressor is released from the
operator because the inducer (allolactose) is
present
 cAMP levels are high because glucose is
absent ,so CAP is active and bound to the
DNA . CAP helps RNA polymerase bind to the
promoter , permitting high levels of
transcription.