The lac operon:
We will consider in depth the regulation of the lac operon where both a
repressor and an activator operate to precisely control the expression of these genes.
This is an
excellent example for how a cell responds to changes in the environment and how a cell can
integrate signals into one response.
Lactose is a disaccharide made up of glucose and galactose.
E. coli prefers to use glucose as its carbon source, but if glucose isn’t around, it can utilize
lactose as its carbon source.
Thus, genes that encode proteins necessary for lactose utilization
are regulated such that when glucose is absent and lactose is present, they are turned on and
when lactose is absent they are turned off.
Before we consider the gene structure and function of the lac operon, we need to remind
ourselves how genes are organized in bacteria.
Coordinately expressed genes are grouped into
Operons are clusters of genes that function together and thus are regulated the same
Transcription from an operon results in the production of a single polycistronic mRNA.
more than one protein is translated from a single mRNA.
The stop codon of one polypeptide is
positioned close to the start codon of the next polypeptide and therefore, translation can continue.
However, consider the consequence of a nonsense mutation in the first gene of an operon.
Because translation will terminate, the ribosome will become dissociated and genes further down
in the operon will not be expressed.
Therefore, a nonsense mutation will have a polar affect on
downstream genes. (
Consider what would happen when performing complementation tests in this
There are three gene encoded by the lac operon:
lacZ, lacY and lacA.
LacZ encodes the
enzyme beta-galactosidase; this enzyme converts lactose to glucose and galactose.
In addition to
this reaction, beta-galactosidase also converts lactose to allolactose, an isomer of lactose and the
natural inducer of the system.
LacY encodes a permease that transports lactose into the cell and
lacA encodes a transacetylase.
Surprisingly, the function of this last gene is not really
understood and therefore will not be discussed further.
In addition to these structural genes, the
operon contains a promoter (the cis acting signals that direct RNA polymerase to initiate
transcription) and an operator, this is a sequence that is the binding site for the repressor of the
LacI is expressed from its own promoter.
In the absence of lactose, there is very low expression from the operon, ~ 3-4 molecules
of beta-galactosidase are present in the cell.
However, within 3-5 min of sensing lactose in the
environment, ~ 5000 molecules of beta-galactosidase are present.
Once lactose is depleted from
the environment, transcription is no longer induced.
The mRNA has a short half life and is
therefore quickly depleted.
Why do you think the cell has any molecules of beta-galactosidase
present in the absence of lactose?
This is necessary to form the inducer allolactose, since beta-