The Lac Operon
The lactose operon (Figure 4-1) provides a good model system of several
concepts of prokaryotic gene regulation. It consists of three structural
, and lacA
) as well as three controlling sites (lacCRP
, and lacO
). The structural genes lacZ
, and lacA
enzymes β-galactosidase, permease, and transacetylase, respectively.
Catabolism of lactose is dependent upon these proteins. The controlling
, and lacO
are binding sites for the cAMP receptor
protein, RNA polymerase, and the lactose repressor, respectively.
The lactose regulatory operon consists of one structural gene (lacI)
and one controlling site (lacP2). The structural gene encodes the lactose
repressor, whereas the controlling site is an RNApolymerase binding site
In the absence of inducer, something that turns on transcription of
an operon, expression is inhibited by the binding of the lactose repressor
at lacO. The repressor sterically hinders the binding of RNApolymerase
to lacP1 and the inititaion of transcription. If lactose is present, it can be
converted by the cell into allolactose, which acts as an inducer for this
operon. When inducer is present, it binds to the repressor (protein lacI)
and inactivates it. Inactive lacI cannot bind the operator, and RNApolymerase
is able to bind to lacP1 and initiate transcription of the genes necessary
for lactose catabolism.
Even when an operon is induced the cell does
not rapidly fill with mRNAand protein since mRNA
has an average half-life of only 2.5 minutes. That
means that 2.5 minutes after mRNA is synthesized,
half of it will be degraded. Proteins are more stable
than mRNA. Also, as the cell synthesizes mRNA
and proteins, it depletes its stores of certain energy
molecules. When a cell is rapidly metabolizing,
catabolite repression shuts down many catabolic operons, including the
lactose operon. This involves a small molecule called cyclic adenosine
monophosphate (cAMP). The cellular level of cAMP decreases when
the synthesis of lactose mRNA and enzymes increases, and the level of
cAMPincreases when these catabolic genes are no longer expressed. The
higher the cAMP level, the more cAMP binds to a protein called cyclic
AMP receptor protein (CRP), which then undergoes a conformational
change that promotes binding to an activator binding site (lacCRP). This
in turn upregulates transcription of the lactose operon genes.
Because the lactose operon is repressed by a regulatory
protein, it is said to be negatively controlled.
Other operons are positively controlled,
that is regulated by proteins that activate the operon.
Lactose present = operon on
Lactose absent = operon off