Biology Reference
In-Depth Information
proC
i
p
o
z
y
a
DNA
Repressor
p
proC
i
o
z
y
a
DNA
RNA polymerase
mRNA
β
-galactosidase
Deactivated
repressor
Proteins
Permease
Acetylase
Figure 4
Control of expression in the lac operon.
We depict the three structural genes (z,y,a) and the control regions (i,o,p) on strands of DNA.
The i region is the structural gene encoding the repressor, p is the promotor region, and o is
the operator region bound by the repressor. In the top figure, the active repressor is present, and
synthesis is inhibited. In the bottom figure, an inducer alters the shape of the repressor so that it
no longer binds the operator, the RNA polymerase binds to the promotor region, and synthesis of
the three structural genes proceeds. The z gene synthesizes mRNA for -galactosidase, the y gene
synthesizes mRNA for permease, and the a gene synthesizes mRNA for acetylase. The mRNA's
are translated into proteins.
This switching behavior of
E. coli
, responding to the variation in sugar sources
available in the environment is the fundamental behavior Jacob andMonod sought
to explain. Strictly, of course, the
lac
operon model is a model of one component in
the metabolic machinery of
E. coli
, though that component is crucially important
in the behavior. In his Nobel lecture, Monod says that in 1947,
it became clear to me that this remarkable phenomenon was almost entirely
shrouded in mystery. On the other hand, by its regularity, its specificity, and by
the molecular-level interaction it exhibited between a genetic determinant and a
chemical determinant, it seemed of such interest and of a significance so profound
that there was no longer any question as to whether I should pursue its study.
(1966, p. 476)
The problem Monod identifies is the 'respective roles of the inducing substrate
and of the specific gene (or genes) in the formation and the structure of the
enzyme' (ibid.).
