Genetics 6 Sections19 LessonsLifetime
The Lac Operon
The Bacterial Operons
Transcription of mRNA
In bacteria, the structural genes that code for proteins involved in a particular metabolic pathway are often found sequentially grouped on the chromosome along with the cis-acting regulatory elements that determine the transcription of these genes.
The result of the transcription is a single polycistronic mRNA.
The genes are coordinately controlled (turned ON or OFF as a unit).
This entire unit is referred to as an “Operon”.
Role of Operators in Prokayotic Transcription
The “operons” in Prokaryotes contain a segment of DNA that regulates the activity of the structural genes of the operon.
This segment is known as an “Operator“.
If the operator is not bound by a repressor molecule – RNA polymerase passes over the operator and reaches the protein-coding genes which it transcribes to mRNA.
If a repressor molecule is bound to the operator, the polymerase is blocked & does not produce mRNA.
No proteins are made as long as the repressor is bound to the operator.
However, when an inducer molecule is present, it binds to the repressor, causing the repressor to change shape so that it no longer binds the operator.
When this happens, the RNA polymerase can proceed with transcription.
This is best understood in the lactose operon of E. coli which illustrates both positive & negative regulation.
The Lactose Operon
The Lactose (lac) operon contains the genes that code for three proteins involved in the catabolism of the disaccharide lactose.
- The lacZ gene codes for β-galactosidase, which hydrolyzes lactose to galactose and glucose
- The lacY gene codes for a permease, which facilitates the movement of lactose into the cell, and
- The lacA gene codes for thiogalactoside transacetylase, which acetylates lactose.
Bacteria use glucose, if available, as a fuel in preference to any other sugar.
All of these proteins are maximally produced only when GLUCOSE IS ABSENT and lactose is available to the cell.
The regulatory portion of the operon is upstream of the 3 structural genes and consists of the promoter region where RNA polymerase binds and two additional sites, the operator (O) site & the CAP site, where regulatory proteins bind.
The lacZ, lacY, and lacA genes are expressed only when:
- The O site is empty, and the CAP site is bound by a complex of cAMP and the catabolite activator protein (CAP), sometimes called the cAMP regulatory protein (CRP).
There is another regulatory gene, the lacI gene.
It codes for the repressor protein (a trans-acting factor) that binds to the O site with high affinity.
The lacI gene has its own promoter.
When Glucose is available
When Glucose is available, the lac operon is repressed–turned off.
Repression is mediated by the repressor protein binding via a helix-turn-helix motif to the operator site, which is downstream of the promoter region.
Binding of the repressor interferes with the progress of RNA polymerase and it cannot proceed.
This blocks transcription of the structural genes.
This is an example of negative regulation.
When Only Lactose is available (Glucose absent)
In this case when only lactose is available and glucose is absent, the lac operon is induced – turned ON and is maximally expressed.
A small amount of lactose is converted to an isomer, ‘Allolactose’.
This compound is an inducer that binds to the repressor protein, changing its conformation so that it can no longer bind to the operator.
In the absence of glucose, adenylyl cyclase is active, and sufficient quantities of cAMP are made and bind to the CAP protein.
The cAMP–CAP trans-acting complex binds to the CAP site, causing RNA polymerase to more efficiently initiate transcription at the promoter site (positive regulation).
The transcript is a single polycistronic mRNA molecule that contains three sets of start and stop codons.
Translation of the mRNA produces the three proteins that allow lactose to be used for energy production by the cell.
In contrast to the inducible lacZ, lacY, and lacA genes, whose
expression is regulated, the lacI gene is constitutive.
Its gene product, the repressor protein, is always made and is active unless the inducer is present.
If glucose is present, transcription of the lac operon is negligible, even if lactose is present at a high concentration.
Adenylyl cyclase is inhibited in the presence of glucose (catabolite repression) so NO cAMP–CAP complex forms, and the CAP site remains empty.
RNA polymerase is, therefore, unable to effectively initiate transcription, even though the repressor may not be bound to the operator region.
As a result the 3 structural genes of the operon are NOT expressed.
Tryptophan and lac operons are the most evil topics in the whole biochemistry and its a piece of cake here. Admirable efforts made