The first control system for enzyme production worked out at the molecular
level described the control of enzymes that are produced in response to the
presence of the sugar lactose in E.
coli cell. The work was performed
by Jacob and Monod for which they were awarded the Nobel Prize. The following is
the pathway that leads to the production of glucose and galactose.
Lactose -----------------------------------> Glucose + Galactose
ß-galactosidase
Several proteins involved in lactose metabolism in the E.
coli cell. They are:
- ß-galactosidase - converts lactose into glucose and galactose
- ß-galactoside permease - transports lactose into the cell
- ß-galactoside transacetylase - function unknown
Research with this system was greatly added by the availability ofconstitutive mutants.
A constitutive mutant is one in which the gene product is produced continually,
that is there is no control over its expression. In these mutants, the above
proteins were produced all the time in comparison to the wild type where the
proteins only appeared in the presence of lactose. So in these mutants, the
mutation must be a gene other than those responsible for the structural genes.
All of the genes involved in controlling this pathway are located next to
each other on the E. coli chromosome.
Together they form an operon. The following is the genetic structure of the
operon.
Control Circuit for the lac Operon
I P O || Z | Y | A |
_________________________________________________________
Controlling || Structural genes
Region
lac Operon
Gene |
Gene Function |
I
|
Gene for repressor
protein |
P
|
Promoter |
O
|
Operator |
lac Z
|
Gene for ß-galactosidase |
lac Y
|
Gene for ß-galactoside
permease |
lac A
|
Gene for ß-galactoside
transacetylase |
Operon -
a cluster of structural genes that are expressed as a group and their associated
promoter and operator
How does the system work? Without
lactose in the cell, the repressor protein binds to the operator and prevents
the read through of RNA polymerase into the three structural genes. With lactose
in the cell, lactose binds to the repressor. This causes a structural change in
the repressor and it loses its affinity for the operator. Thus RNA polymerase
can then bind to the promoter and transcribe the structural genes. In this
system lactose acts as an effector molecule.
Effector molecule -
a molecule that interacts with the repressor and affects the affinity of the
repressor for the operator
With the above information, we can
now predict the effect that various mutants will have on lac operon
gene expression.
Mutant lacgene |
Mutant Phenotype |
I-
|
constitutive expression
because the operator is never closed |
O-
|
constitutive expression
because the repressor protein can not bind |
P-
|
no expression of the
operon because RNA polymerase cannot bind |
lac Z-
|
no glucose or galactose
production from lactose |
lac Y-
|
no induction because
lactose will not be taken into the cell |
Catabolite Repression of the lac Operon
Lactose is not the preferred carbohydrate source for E.
coli. If lactose and glucose are present, the cell will use all of the
glucose before the lac operon
is turned on. This type of control is termed catabolite
repression. To prevent lactose metabolism, a second level of control of gene
expression exists. The promoter of the lac operon
has two binding sites. One site is the location where RNA polymerase binds. The
second location is the binding site for a complex between the catabolite
activator protein (CAP) andcyclic
AMP (cAMP). The binding of the CAP-cAMP complex to the promoter site is
required for transcription of the lac operon.
The presence of this complex is closely associated with the presence of glucose
in the cell. As the concentration of glucose increases the amount of cAMP
decreases. As the cAMP decreases, the amount of complex decreases. This decrease
in the complex inactivates the promoter, and the lac operon
is turned off. Because the CAP-cAMP complex is needed for transcription, the
complex exerts a positive control over
the expression of the lac operon.