294
JEAN BRÄCHET
(presumably messenger) RNA begins. The phenotypic result must be
the synthesis of specific proteins, including probably the inducing agents.
There is no doubt that, in the fully grown ovocyte and in the
unfertilized egg, protein and RNA synthesis both come to a standstill :
their ribosomes obviously become metabolically inactive. There are
many possible reasons for this inhibition, or repression, of ribosomal
activity. For example, the RNA itself might be altered, since it has been
shown by Finamore and Volkin (1961) that ribonucleoproteins and
RNA, in fully grown frog ovocytes, show an abnormal solubility in
NaCl and perchloric acid respectively. Or it may be that ribosomes, in
adult ovocytes and unfertilized eggs, have their active templates
blocked by this abnormal RNA or, in a less specific way, by latent
ribonuclease or histones. Finally, ribosomes might be inactive simply
because they lack messenger RNA which is no longer produced by the
nucleus.
It is impossible to give preference to any of these possibilities until
they are experimentally tested. While the suggestion, which has already
been discussed in the case of Acetabularia, that ribosomes are inactive
because they no longer receive messenger RNA, has the merit of greater
simplicity, the inhibition (or repression) hypothesis explains a number
of facts in a relatively easy manner.
For instance, this hypothesis would be in agreement with the recent
observations of Hultin (1961), who found that, in sea urchin eggs, the
incorporation of amino-acids into proteins shows a very rapid increase
immediately after fertilization both in vivo and in vitro : obviously, the
ribosomes of the unfertilized eggs are somehow inactivated.f This
hypothesis that ribosomes are 'inhibited' or 'repressed' could also
explain some phenomena associated with induction ; thus both induction
by the normal organizer and spontaneous neuralization by acid or
alkaline shocks can be accounted for on the assumption that inducing
agents or precytolytic agents lift the ribosomal block, permitting
ribosomes to accept messenger RNA, which is supposed, at this stage of
development, to be produced by the nuclei. In fact, such a hypothesis is
a revival of the old 'unmasking' hypothesis of Waddington et al. (1936) :
the basic idea, i.e., that the induction liberates a neuralizing factor from
an inactive complex, remains fundamentally the same. Recent work by
Tiedemann et al. (1961) is in agreement with such a view: according to
them, in normal induction, both neuralizing and mesodermizing factors
f However, M. Nemer (1962) has recently shown that addition of synthetic polynucleotides strongly stimulates the incorporation of amino-acids into the inactive ribosomes of
unfertilized sea urchin eggs : this brings strong evidence for the view that their inactivity
is simply due to the lack of messenger RNA, although the possibility that the synthetic
polynucleotides might combine with a ribosomal inhibitor, and thus lift the inhibition,
cannot yet be ruled out.
JEAN BRÄCHET
(presumably messenger) RNA begins. The phenotypic result must be
the synthesis of specific proteins, including probably the inducing agents.
There is no doubt that, in the fully grown ovocyte and in the
unfertilized egg, protein and RNA synthesis both come to a standstill :
their ribosomes obviously become metabolically inactive. There are
many possible reasons for this inhibition, or repression, of ribosomal
activity. For example, the RNA itself might be altered, since it has been
shown by Finamore and Volkin (1961) that ribonucleoproteins and
RNA, in fully grown frog ovocytes, show an abnormal solubility in
NaCl and perchloric acid respectively. Or it may be that ribosomes, in
adult ovocytes and unfertilized eggs, have their active templates
blocked by this abnormal RNA or, in a less specific way, by latent
ribonuclease or histones. Finally, ribosomes might be inactive simply
because they lack messenger RNA which is no longer produced by the
nucleus.
It is impossible to give preference to any of these possibilities until
they are experimentally tested. While the suggestion, which has already
been discussed in the case of Acetabularia, that ribosomes are inactive
because they no longer receive messenger RNA, has the merit of greater
simplicity, the inhibition (or repression) hypothesis explains a number
of facts in a relatively easy manner.
For instance, this hypothesis would be in agreement with the recent
observations of Hultin (1961), who found that, in sea urchin eggs, the
incorporation of amino-acids into proteins shows a very rapid increase
immediately after fertilization both in vivo and in vitro : obviously, the
ribosomes of the unfertilized eggs are somehow inactivated.f This
hypothesis that ribosomes are 'inhibited' or 'repressed' could also
explain some phenomena associated with induction ; thus both induction
by the normal organizer and spontaneous neuralization by acid or
alkaline shocks can be accounted for on the assumption that inducing
agents or precytolytic agents lift the ribosomal block, permitting
ribosomes to accept messenger RNA, which is supposed, at this stage of
development, to be produced by the nuclei. In fact, such a hypothesis is
a revival of the old 'unmasking' hypothesis of Waddington et al. (1936) :
the basic idea, i.e., that the induction liberates a neuralizing factor from
an inactive complex, remains fundamentally the same. Recent work by
Tiedemann et al. (1961) is in agreement with such a view: according to
them, in normal induction, both neuralizing and mesodermizing factors
f However, M. Nemer (1962) has recently shown that addition of synthetic polynucleotides strongly stimulates the incorporation of amino-acids into the inactive ribosomes of
unfertilized sea urchin eggs : this brings strong evidence for the view that their inactivity
is simply due to the lack of messenger RNA, although the possibility that the synthetic
polynucleotides might combine with a ribosomal inhibitor, and thus lift the inhibition,
cannot yet be ruled out.