Biology Reference
In-Depth Information
other regulates the importation of lactose into the cell. For Jacob and Monod
this was not yet clear.)
Jacob and Monod found that some lac
−
organisms could be persuaded to
produce -galactosidase by inducers other than lactose, though this did not mean
they used lactose in metabolism. This entailed that these lac
−
mutants lacked a
factor the wild types had - one which would allow lactose to be brought into the
cell. We now see this as the
lacY
gene. As a result, we know the structural genes
regulated within the operon included at least two with very different functions.
One was involved in bringing lactose into the cell, and one was responsible for
producing -galactosidase. To recapitulate, one mutant consumed lactose, taking
it up from the environment, but could not metabolize it; the other could not
take up lactose from the environment, and could have metabolized it if it had
taken it in, as it produced -galactosidase. The actual mechanism of control was
still uncertain. More specifically, these results left open the question whether
lactose consumption in the 'wild type' forms is triggered by stimulation or by
inhibition of repression. Monod later described this as a 'double bluff', asking
'Why not suppose, then, that induction could be effected by an anti-repressor
rather than by repression by an anti-inducer' (1966, p. 479). This gave two
different pictures of possible mechanisms behind biphasic growth. On the one
hand, genetic control could be mediated by a switch that facilitates transcription
of -galactosidase. On the other hand, genetic control could be mediated by a
switch that inhibits the production of -galactosidase and can be turned off by
a repressor.
The experimental solution was in bacterial sex (conjugation). Crossing lac
+
and lac
−
in some cases changed the acceptor lac
−
into a lac
+
organism, which
meant that there had to be some factor available which repressed, or inhibited,
expression of the gene. In other words, a bacterium which was incapable of
limiting lactose consumption became capable of inhibiting lactose consumption,
having received some factor from the lac
+
'wild type' form. Shortly after this
discovery, in 1960, Monod first introduced the term 'operon' to describe the
coordinate expression of the
lacY
and
lacZ
genes. At this point, he knew only
that the Y and Z genes were regulated by a promotor and an operator (POZY).
Jacob and Monod, in the following years, promoted this as a general model
of gene regulation (see Jacob 1966, Monod 1966, Jacob & Monod 1961, and
Beckwith 1967).
What we have seen is the idea that there had to be a control mechanism to
explain the ability of
E. coli
to switch food sources, which could (theoretically)
have been subject to either stimulation or repression, and they settled on repres-
sion. Moreover, we have seen how they revealed the structure and organization
of both regulatory elements and the structural genes they control, based on exper-
imental evidence. What was most notably lacking was experimental evidence
concerning the nature of the repressor.
