the lac operon
TRANSCRIPT
THE lacOPERON
Molecular BiologyLecture No: MB 12
Dr. Aga Syed SameerCSIR Lecturer (Demonstrator)
Department of Biochemistry,
Medical College,
Sher-I-Kashmir Institute of Medical Sciences,
Bemina, Srinagar, Kashmir, 190018. India.
Regulation of Gene Expression
Each cell has almost all the genetic material for its growth and development
Some genes will be need to be expressed all the time : are involved in vital biochemicalprocesses such as respiration, metabolism etc
Other genes need not to be expressed all the time: are switched on an off at need
Operons
The Operon concept was given by Jacob, Monod and Lwoff in 1961
An operon is a group of genes that are transcribed at the same time (hence, expression also)
They usually control an important biochemical process
They are only found in prokaryotes
Operons The operon is actually a unit of gene expression &
regulation which consists of:
Structural genes: which code for the certain specific
enzymes involved in metabolic pathways. Their expression is coordinately controlled by various mechanisms
Operator & Promoter (OP) Sequences: which are the control
sequences located upstream of the structural genes and
which in turn play role in regulating the transcription of the
structural genes
Regulator gene: which code for the protein that binds to the OP Sequences, to control the structural genes
transcription/expression
The lac Operon
The lac operon consists of three functional units:
Structural genes: Lac ZYA
Operator/Promoter Sequences: Lac O and Lac P
Regulator gene: Lac I
The lac Operon
lac Y
The lac Operon
The three structural genes are coded as polycistronic mRNA, which in turn code for three polypeptides
One of them is enzyme β-galactosidase -which hydrolyses lactose into glucose and galactose
Second is Permease - which transports lactose within the cell via its internal channel
Third one has unknown function
Adapting to the environment
E. coli can use either Glucose (monosaccharide) or Lactose (disaccharide) for its energy requirements
However, Lactose needs to be hydrolysed (digested) first
So the bacterium prefers to use Glucose when/wherever it can (from environment)
Regulation of Lac Operon
Molecular Aspect Lac I Repressor: Coded by Lac I gene
binds to Operator and prevents the
Transcription of Lac ZYA.
This occurs in complete absence of
Lactose /in case of well fed Glucose
state.
Molecular AspectCAP Activator: It binds to the promoter
region of the Lac operon and causes 90o
kink in DNA and hence enhances the
promoter binding of the RNA Pol.
This occurs in complete absence of
Glucose as well is case of starved state
Lactose as sole source of energy
Glucose:
Low or Absent
cAMP High cAMP dimerises CAP
CRP/CAP attaches to promoter
Expression of Lac ZYA enhances
β-galactosidase ↑ in cell
Permease ↑ in membrane
Lactose will be utilised
Lactose:
Low or Absent
LacI being synthesised
LacI Repressor attaches to
operator
Expression of Lac ZYA is
halted
β-galactosidase ↓ in cell
Permease ↓ in membrane
Glucose will be utilised
Molecular Aspect
Four situations are possible1. 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
Thus
β-galactosidaseis synthesized only in
presence of Lactose
(As a sole source of carbon)
1. When lactose is absent
A repressor protein is continuously synthesised. It sits on a sequence of DNA just in front of the lacoperon, the Operator site
The repressor protein blocks the Promoter site where the RNA polymerase settles before it starts transcribing
Regulator
genelac operon
Operator
site
z y aDNA
I O
Repressor
protein
RNA
polymeraseBlocked
2. When lactose is present
A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the repressor protein at another active site (allosteric site)
This causes the repressor protein to change its shape (a conformational change). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site
z y a
DNA
I O
2. When lactose is present
A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the repressor protein at another active site (allosteric site)
This causes the repressor protein to change its shape (a conformational change). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site
Promotor site
z y aDNA
I O
3. When both glucose and lactose are present
This explains how the lac operon is
transcribed only when lactose is present.
BUT….. this does not explain why the
operon is not transcribed when both
glucose and lactose are present.
When glucose and lactose are present RNA
polymerase can sit on the promoter site but
it is unstable and it keeps falling off
Promotor site
z y aDNA
I O
Repressor protein
removed
RNA polymerase
4. When glucose is absent and
lactose is present Another protein is needed, an catabolite activator
protein. This stabilises RNA polymerase.
The catabolite activator protein only works when glucose is absent.
As cAMP is available only in absence of glucose. In this way E. coli only makes enzymes to metabolise
other sugars in the absence of glucose.
Promotor site
z y a
DNAI O
Transcription
Activator
protein steadies
the RNA
polymerase
SummaryCarbohydrates Activator
protein
Repressor
protein
RNA
polymerase
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
Questions?