Biology Reference
In-Depth Information
regulate gene activity, act. The work of Monod and Jacob on the
regulation of genes involved in lactose metabolism provided an
answer (Jacob and Monod, 1961).
When
Escherichia coli
bacteria grow in a medium without
lactose, the enzymes that allow it to be metabolised are not syn-
thesised. If lactose is added to the culture medium, their synthesis
increases greatly in less than two minutes. This regulation permit-
ting the bacterium to adapt very effectively to its environment
occurs directly at the level of gene activity.
Three structural genes code for the enzymes of lactose metabo-
lism. They are grouped together on a single portion of DNA called
the lactose operon (Fig. 7). A regulator gene
i
codes for a repressor
protein
R
which represses the activity of the three genes on the
operon. Regulation of this operon involves two other portions of
DNA, the operator
o
and the promoter
p
, situated upstream of
the lactose genes.
R
binds stereospecifically to the operator
o
or to
lactose, but these bonds are mutually exclusive. In the absence of
lactose (Fig. 7A),
R
fixes on
o
and the enzyme, RNA polymerase,
which transcribes the lactose genes, is prevented from passing
beyond the promoter
p
. In the presence of lactose (Fig. 7B),
R
binds
with it liberating the site
o
. Transcription of the genes can then
resume.
Other regulatory models have been described for
Escherichia
coli
. In some cases, the regulator protein is an activator that stim-
ulates gene activity. In all these models, however, very precise reg-
ulation which excludes chance is thought to be due to the property
of stereospecific recognition between the molecules. Because regula-
tor signals act according to an “all or nothing” rule, the genes are
either active or repressed.
This regulation logic has been generalised since to the overall
way in which the genome functions in multicellular organisms
(Monod and Jacob, 1961). The differential activity of the genes in
the cell lineages of an organism has been explained by cascade reg-
ulation similar to that of the lactose operon. Indeed, in this model,
if one of the genes of an operon codes for a regulator protein capa-
ble of controlling the activity of other operons in the same genome,
