290
JEAN BRÄCHET
hypothesis, which would also explain the fact that mercaptoethanol is
more active in inhibiting gastrulation and neurulation than cleavage
(during which the basic proteins are absent from the nuclei). It would be
of special importance to know whether lipoic acid also combines
preferentially with the basic proteins of developing amphibian eggs.
But other possibilities should be kept in mind, in particular an effect of
mercaptoethanol and lipoic acid on the lipid constituent of the egg (cell
membrane, constituents of the membrane of the endoplasmic reticulum,
Golgi apparatus, etc.).
The uptake and incorporation of labelled mercaptoethanol have also
been studied in Acetabularia. Using a concentration of M/300,|it was found
that the uptake of
35
/S
r -mercaptoethanol increases during a 1 week period ;
however, the uptake is by far the greatest during the first 3 days of
treatment and the algae are almost saturated with mercaptoethanol at
that time. The proteins are quickly labelled, a maximum being reached
after the end of the first day ; the radioactivity of the proteins corresponds to 10% of the total. However, if the algae are very strongly illuminated, as much as 20% of the radioactivity is found in the protein fraction.
Further experiments have shown that the radioactivity of the rhizoid
is some 50% higher than that of the non-nucleate stems. In experiments
made on homogenates, it was observed that the radioactivity is rather
uniformly distributed among the various fractions which can be
recovered by centrifugation (starch granules, chloroplasts, mitochondria,
cell walls and ribosomes). There is very little labelling of those proteins
which remain in the supernatant after ultracentrifugation. In this
respect Acetabularia behaves exactly like the amphibian eggs.
These biochemical findings have been substantiated by autoradiographic studies of
35
#-mercaptoethanol-treated algae, which led to the
interesting conclusion that there are two different sites for preferential
binding of the labelled thiol to the protein. One is the nucleus, in which
both the nucleolus and the nuclear sap are heavily labelled. The other is
the apex of the stem, i.e., the region where the cap should form. This
distribution is of particular interest, since we have seen that the morphogenetic substances in Acetabularia
are produced by the nucleus and
accumulated in the tip, which acts as a receptor site for these substances.
It is a striking fact that mercaptoethanol is preferentially bound, in
insoluble form, in the two regions of greatest morphogenetic importance
in Acetabularia as well as in amphibian eggs.
V. Discussion
Considering first the relatively simple case of Acetabularia, it can be
concluded that DNA replication is not a prerequisite of morphogenesis.
JEAN BRÄCHET
hypothesis, which would also explain the fact that mercaptoethanol is
more active in inhibiting gastrulation and neurulation than cleavage
(during which the basic proteins are absent from the nuclei). It would be
of special importance to know whether lipoic acid also combines
preferentially with the basic proteins of developing amphibian eggs.
But other possibilities should be kept in mind, in particular an effect of
mercaptoethanol and lipoic acid on the lipid constituent of the egg (cell
membrane, constituents of the membrane of the endoplasmic reticulum,
Golgi apparatus, etc.).
The uptake and incorporation of labelled mercaptoethanol have also
been studied in Acetabularia. Using a concentration of M/300,|it was found
that the uptake of
35
/S
r -mercaptoethanol increases during a 1 week period ;
however, the uptake is by far the greatest during the first 3 days of
treatment and the algae are almost saturated with mercaptoethanol at
that time. The proteins are quickly labelled, a maximum being reached
after the end of the first day ; the radioactivity of the proteins corresponds to 10% of the total. However, if the algae are very strongly illuminated, as much as 20% of the radioactivity is found in the protein fraction.
Further experiments have shown that the radioactivity of the rhizoid
is some 50% higher than that of the non-nucleate stems. In experiments
made on homogenates, it was observed that the radioactivity is rather
uniformly distributed among the various fractions which can be
recovered by centrifugation (starch granules, chloroplasts, mitochondria,
cell walls and ribosomes). There is very little labelling of those proteins
which remain in the supernatant after ultracentrifugation. In this
respect Acetabularia behaves exactly like the amphibian eggs.
These biochemical findings have been substantiated by autoradiographic studies of
35
#-mercaptoethanol-treated algae, which led to the
interesting conclusion that there are two different sites for preferential
binding of the labelled thiol to the protein. One is the nucleus, in which
both the nucleolus and the nuclear sap are heavily labelled. The other is
the apex of the stem, i.e., the region where the cap should form. This
distribution is of particular interest, since we have seen that the morphogenetic substances in Acetabularia
are produced by the nucleus and
accumulated in the tip, which acts as a receptor site for these substances.
It is a striking fact that mercaptoethanol is preferentially bound, in
insoluble form, in the two regions of greatest morphogenetic importance
in Acetabularia as well as in amphibian eggs.
V. Discussion
Considering first the relatively simple case of Acetabularia, it can be
concluded that DNA replication is not a prerequisite of morphogenesis.