2. LAC-OPERON:
The elucidation of the lac operon was a result of a close association between a
geneticist, FRANCOIS JACOB and a biochemist, JACQUE MONOD.
In Lac-Operon,a polycistronic structural gene is regulated by a common promoter and
regulatory genes and such arrangement is called Operon, which are common in
Bacteria.
The Lac Operon consists of
1. Structural Genes :There are 3structural genes namely z, y, a.
Lac z gene codes for β-Galactosidase, splits lactose into glucose and
galactose.
The y genes codes for Galactoside permease facilitates the entry of lactose into
cell .
The a gene codes for Galactoside transacetylase appears in small quantities
upon Lactose Induction.
3. 2]Operator gene: The Operator gene is adjacent to the first structural gene.
It determines whether or not the structural genes are to be repressed by the
repressor. The operator is recognized by the repressor protein which binds to
the operator forming an operator-repressor complex.
3]Promoter gene: RNA Polymerase binds to DNA at the promoter site and
increases transcription and protein synthesis.
4]Regulatory Gene: There is only one regulatory gene called i-gene, it is
derived from the word inhibitor.
The regulatory gene directs the synthesis of a protein which may be an
active repressor or an inactive repprossor or also called as aporepressor.
The control of protin synthesis by regulator protein may be by induction or
repression .they may also negative or positive control.
4. INDUCTION:
Inducible enzymes absent in cell,they synthesize and increase in
quantity only in the presence of an inducer,such a system is called as inducible
system.
Abacterial culture of E.Coli growing in lactose medium the production of
Beta –Galactosidase starts and within 2 mins about 3000 molecules are
synthesized by z gene,by which galactose and glucose are formed.
Some glucode and galactose molecules are converted into lactose and allo
lactose is the Inducer for for Beta –Galactosidase synthesizes.
When the Inducer is absent the active repressor protein produced by the
regulator gene binds to the operator region and prevents RNA polymerase from
transcribing the operon.
When the Inducer is present the active repressor binds to the inducer molecule
to form a REPRESSOR-INDUCER Complex.A confirmational change takes place in
the repressor molecule, which is inactivated .This prevents binding to the
operator,therefore it is not blocked.the structural genes transcribes Lac mRNA
and enzyme synthesis takes place.
5.
6. NEGATIVE CONTROL OF LAC OPERON
It has been seen that both in inducible and repressible systems.
Protein synthesis takes place when the operator gene is free and stops when
it is blocked.
Gene expression therefore only takes place when operator gene is free.such a
control mechanisms for protein synthesizes is of negative type.
Ex:the induction of protein synthesis in presence of lactose by the Lac Operon
of E.Coli.
In Negative control,the regular protein and its it prevents gene transcription.
In INDUCIBLE SYSTEM,the effector molecule is the inducer .
The inducer prevents repressor function and this enables gene
transcription ,protein synthesis.
In REPRESSIBLE SYSTEMS,the effector molecue is co-repressor stimulates
repressorand prevents gene transcription.
7. POSITIVE CONTROL OF LAC OPERON
It has been seen in Inducible system only
In positive control, the regulator protein is called activators.
Protein synthesis takes place when the activator bind to operator region and
stops in the absence of activator.
EX:Catabolite Sensitive Operons of E.Coli.
In Inducible system, the activator is inactive state and cannot bind DNA.When
an inducer molecule interacts with the activation the activator becomes
active and binds,these permites transcription of operon.
8.
9. Catabolite Sensitive Operons
These operon are concerned with utilization of those energy sources that are
used only when glucose is absent from medium .
The effect of catabolites on these operons is mediated by CAMP-Cyclic
Adenosine Monophosphate ,which acts via a protein called CATABOLITE
ACTIVATOR PROTEIN[CAP] .
These operons are transcribed only when CAP binds to a region of their
promoters but CAP can bind DNA only when it is associated with CAMP.
When glucose is absent from medium,the cells have CAMP above a threshold
concentration,therefore CAP interacts with CAMP and binds to DNA this
enables transcription of catabolite sensitive operons.
In presence of glucose in the medium ,it is preferentially used by cells.This is
because glucose reduces CAMP level in the cell below the threshold level
needed for CAP activation.As a result ,CAP is unable to bind the promoter of
‘catabolite sensitive operons and they are not transcribed.
10. Inducible positive control of Lac Operon
of E.Coli:
1]CAP interacts with cyclic AMP,this allows CAP and RNA Polymerases to bind
to the promoter and begin transcription.
2]In the absence of cyclic AMP,CAPis unable to bind to the promoter.As a
result,RNA Polymerase cannot attach to the promoter and transcription
cannot begin evin if the operator is free from the repressor.
