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JEAN BRÄCHET
appearance, in the progeny, of a modified protein. This modification of
the protein is perpetuated in the absence of the Y-chromosome and is
not dependent upon the cytoplasm. As in Acetabularia, there must exist
a recipient of genetic material. Fox et al. (1962) suggest that there might
be an enzyme which catalyzes the synthesis of RNA, but using preformed RNA (not DNA) as a primer. Such an enzyme has been recently
described by Nakamoto and Weiss (1962) in bacteria and it would be of
considerable interest to know whether it is present in
Acetabularia.
Another possibility is that the RNA which is produced by the Acetabularia
nucleus is not messenger RNA but a direct precursor of ribosomal RNA;
such an RNA apparently exists in yeast, according to Kitazume et al.
(1962). Finally, as we shall see, one cannot even exclude the possibility
that the morphogenetic substance is a protein rather than an RNA. It is
clear, for example, that the integrity of the sulphur-containing proteins
is required for normal morphogenesis in Acetabularia as in amphibian
eggs. But it is likely that their role in differentiation is more closely
related to the expression of the genetic character (cap formation, for
instance) than to the production of the morphogenetic substances
themselves. While the action of DNA, messenger RNA and, to
a lesser extent, ribosomal and transfer RNA's may result in the
production of morphogenetic substances or inducing agents, proteins
would play an essential role in the reacting system; their molecular
structure should remain intact for a normal reaction to inducing
stimuli.
We have seen that, in amphibian eggs, inducing substances are
probably proteins (basic proteins, according to Vahs, 1962); but RNA
synthesis is required for successful induction as well as cap formation in
Acetabularia (Toivonen et al., 1961; Yamada and Karasaki, 1962). So
long as the 'morphogenetic substances' of Acetabularia have not been
isolated, purified and tested for cap-producing-activity, we cannot rule
out a similar situation in the algae : present experimental evidence can
still be explained by assuming that messenger and ribosomal RNA's are
required for the synthesis of a protein (perhaps a basic protein) which
induces the formation of caps in Acetabularia. If so, the two morphogenetic systems discussed in this chapter would be almost identical:
messenger and ribosomal RNA's would control the synthesis of a
protein with morphogenetic activity ; the integrity of the proteins would
also be required for a successful response of the reacting system to the
morphogenetic stimulus.
The role of nucleic acids in differentiating
embryos remains to be
discussed. It is still too early to present a general theory of differentiation
at the molecular level. But, in this field, hypotheses which can be tested
experimentally are needed. We shall therefore present a few ideas which
JEAN BRÄCHET
appearance, in the progeny, of a modified protein. This modification of
the protein is perpetuated in the absence of the Y-chromosome and is
not dependent upon the cytoplasm. As in Acetabularia, there must exist
a recipient of genetic material. Fox et al. (1962) suggest that there might
be an enzyme which catalyzes the synthesis of RNA, but using preformed RNA (not DNA) as a primer. Such an enzyme has been recently
described by Nakamoto and Weiss (1962) in bacteria and it would be of
considerable interest to know whether it is present in
Acetabularia.
Another possibility is that the RNA which is produced by the Acetabularia
nucleus is not messenger RNA but a direct precursor of ribosomal RNA;
such an RNA apparently exists in yeast, according to Kitazume et al.
(1962). Finally, as we shall see, one cannot even exclude the possibility
that the morphogenetic substance is a protein rather than an RNA. It is
clear, for example, that the integrity of the sulphur-containing proteins
is required for normal morphogenesis in Acetabularia as in amphibian
eggs. But it is likely that their role in differentiation is more closely
related to the expression of the genetic character (cap formation, for
instance) than to the production of the morphogenetic substances
themselves. While the action of DNA, messenger RNA and, to
a lesser extent, ribosomal and transfer RNA's may result in the
production of morphogenetic substances or inducing agents, proteins
would play an essential role in the reacting system; their molecular
structure should remain intact for a normal reaction to inducing
stimuli.
We have seen that, in amphibian eggs, inducing substances are
probably proteins (basic proteins, according to Vahs, 1962); but RNA
synthesis is required for successful induction as well as cap formation in
Acetabularia (Toivonen et al., 1961; Yamada and Karasaki, 1962). So
long as the 'morphogenetic substances' of Acetabularia have not been
isolated, purified and tested for cap-producing-activity, we cannot rule
out a similar situation in the algae : present experimental evidence can
still be explained by assuming that messenger and ribosomal RNA's are
required for the synthesis of a protein (perhaps a basic protein) which
induces the formation of caps in Acetabularia. If so, the two morphogenetic systems discussed in this chapter would be almost identical:
messenger and ribosomal RNA's would control the synthesis of a
protein with morphogenetic activity ; the integrity of the proteins would
also be required for a successful response of the reacting system to the
morphogenetic stimulus.
The role of nucleic acids in differentiating
embryos remains to be
discussed. It is still too early to present a general theory of differentiation
at the molecular level. But, in this field, hypotheses which can be tested
experimentally are needed. We shall therefore present a few ideas which