274 MHR • Unit 3 Molecular Genetics
If lactose molecules are present in the E. coli
cell’s environment, they will be taken up by the
cell and some will bind to the repressor. When this
happens, the repressor detaches from the operator
site. In this case, the lactose molecule serves as an
inducerby stopping the action of the repressor.
As shown in Figure 8.28, RNA polymerase can
then bind to the promoter sequence and begin
transcription. The result is that the enzymes that
allow the cell to break down lactose are synthesized
only in the presence of lactose.Positive Gene Regulation in the lacOperon
The lacgenes code for enzymes that break lactose
down into its component sugars, glucose and
galactose. The E. colicell then uses the glucose as
a source of energy. But what if lactose and glucose
are both available to the cell? In this case, it makes
more sense for the cell to use the glucose that is
already present than to expend energy on breaking
down lactose. A second mechanism in the lac
operon ensures that the lacgenes are expressed at
significant levels only when there is no glucose
available to the cell.
If there is no glucose in its medium, an E. coli
cell will be low on energy. Under these conditions,
the molecule cyclic AMP (or cAMP) will accumulate
in the cell (for a review of cell energy cycle, see
Chapter 3). The accumulation of cAMP triggers
the following series of events, as illustrated in
Figure 8.29 on the following page:
cAMP binds to a protein called an activator(also
called a catabolite activator protein, or CAP).When cAMP binds to the activator, the activator
then binds to a site close to the Placpromoter.
The attachment of the activator makes it easier
for RNA polymerase to bind to the promoter. As
more RNA polymerase binds to the promoter, the
lacgenes are transcribed at a higher rate.
This is an example of positive gene regulation.
In this situation, the direct interaction of a protein
molecule with the genome increases the rate of
gene expression.
The combination of the two types of gene
regulation works rather like the combination of the
ignition switch and the accelerator pedal in a car.
Like the ignition switch, the negative control
function determines whether the gene is “on” or
“off.” The positive control function regulates how
fast transcription occurs once the gene is on, much
like an accelerator pedal. If the ignition is off,
pressing the accelerator pedal has no effect.
Similarly, the E. colicell will produce lacenzymes
only if lactose is available. When lactose is
available, the cell raises or lowers the amount of
lacenzymes produced in accordance with its
actual need for lactose as a nutrient source. The
interaction between these two forms of control is
illustrated in Figure 8.30 on the following page.Co-repression in the trypOperon
In the lacoperon you saw two different examples
of gene regulation. In one, a repressor protein binds
to the genome unless it is removed through the
action of an inducer molecule. In the other, an
activator protein does not bind to the genome unlessFigure 8.28The action of the repressor and inducer in the lacoperon. The
gene that codes for the repressor is not part of the lacoperon, so this protein
is synthesized regardless of whether the lacsystem is active.promoter
repressorrepressorP O Z Y acP O Z Y acP O Z Y acoperator
lactose molecule (inducer)RNA polymerasebeta-galactosidase permease transacetylase
enzymes to be translatedlac operonWhen no lactose is present, the
repressor binds to the operator
and prevents transcription.A
If a lactose molecule binds to the
repressor, the repressor dissociates
from the operator.B
When the operator region is vacant,
RNA polymerase binds to the
promoter and begins transcription.