The lac operon contains three genes - lacZ, lacY, and lacA - that encode proteins involved in lactose metabolism. The activity of the lac operon promoter is regulated by the Lac repressor and CAP proteins. Negative regulation occurs when the Lac repressor binds to the operator region and prevents RNA polymerase from binding and initiating transcription. Positive regulation happens when the CAP-cAMP complex binds near the promoter and stimulates RNA polymerase binding, increasing transcription. Together, these two types of regulation allow the lac operon to be turned on or off depending on the presence of lactose in the environment.
2. INTRODUCTION
LAC OPERON
The lac operon is an operon, or group of genes
with a single promoter . The genes in the operon
encode proteins that allow the bacteria to use
lactose as an energy source.
The lac operon is a cluster of three structural
genes encoding proteins involved in lactose
metabolism and the sites on the DNA involved in
regulation of the operon. The three genes are:
(1) lacZ, which encodes the enzyme β-galactosidase
(2) lacY, which encodes lactose permease
(3) lacA, which encodes a lactose transacetylase.
3. Regulation
The activity of the promoter that
controls the expression of the lac
operon is regulated by two different
proteins ie. Lac repressor and
CAP(Catabolite Activator Protein). One
of the proteins prevents the RNA
polymerase from transcribing (negative
control), the other enhances the binding
of RNA polymerase to the promoter
(positive control).
4. Types of regulation
There are basically 2 types of
regulation in lac operon:-
1. Negative regulation-occurs when
binding of a protein prevents an event.
2. Positive regulation- occurs when
binding of the protein causes a event.
5. Negative regulation
The protein that inhibits transcription of
the lac operon is a tetramer with four identical
subunits called lac repressor.
The lac repressor binds to the operator.
The DNA sequence covered by the repressor
overlaps the DNA sequence recognized by the
RNA polymerase.
Therefor , when the repressor is bound to the
operator , RNA polymerase cannot bind to the
promoter and transcription can not occur ,the lac
operon is said to be under negative control
7. Positive regulation
When the expression of genetic information is quantitatively
increased by the presence of specific regulator elements, it is
called positive regulation.
Catabolite represion is a type of positive control in the lac
operon.
8. The catabolite activator protein (CAP)
,complex cAMP , binds to a site near the
promoter and stimulates the binding of
RNA polymerase.
The cellular level of cAMP are controlled
by glucose ; allolactise level increase the
abundance of cAMP and enhance the
transcription of the abundance of cAMP
and enhance the transcription of the lac
structural gene.
