NUCLEIC ACIDS AND SULPHYDRYL GROUPS
291
In fact, the very presence of DNA is not necessary for morphogenesis
and protein synthesis, since these proceed at a normal rate in anucleate
fragments. On the other hand, RNA is present in large amounts in the
nucleolus and the nuclear sap of Acetabularia.
This nuclear RNA is
synthesized at a rapid rate and the experiments, at our present stage of
knowledge, indicate that it is transferred to the cytoplasm and accumulated in the tip of the alga, which is the site of morphogenesis. In other
words, there is a good deal of evidence, but no definite proof, for the
assumption that, in Acetabularia,
the 'morphogenetic substances'
of
Hämmerling (1953, 1959) could be equated to 'messenger'
{informational)
RNA. Such a simple hypothesis has an additional advantage: it easily
explains the cessation of regeneration and of protein synthesis 2 to 3
weeks after removal of the nucleus. If the Acetabularia nucleus produces
messenger RNA's, the latter must, in the anucleate cytoplasm, have a
relatively long life of 2 to 3 weeks (in contrast to the case of bacteria), or
be capable of independent synthesis in the absence of the nucleus. It is
conceivable that the supply of messenger RNA's in the anucleate
cytoplasm becomes exhausted after 2 to 3 weeks ; although the ribosomes
still exist at that time, their synthetic activity would cease because they
no longer receive messenger RNA's. A comparable situation has been
demonstrated recently in reticulocytes by Arnstein et al. (1962): the
ribosomes of these anucleate cells lose their activity when their
messenger RNA is removed.
It is obvious that the present hypothesis, that the morphogenetic
substances are messenger RNA molecules and that both morphogenesis
and protein synthesis stop, in the absence of the nucleus, when these
molecules have been exhausted, needs a good deal more experimental
support before we can accept it as a fact. Production by the nucleus of
messenger RNA, with a base composition similar to that of Acetabularia
DNA (which has not so far been isolated and analysed), must first be
demonstrated. One should also demonstrate the presence, in the
Acetabularia nucleus, of a DNA-primed RNA polymerase, which is
required to produce messenger RNA (Weiss and Nakamoto, 1961).
Finally, the lifetime of messenger RNA in the anucleate cytoplasm and
its accumulation at the tip of the alga still require experimental
demonstration. For the time being, the advantages of our hypothesis lie
in its simplicity, its agreement with known facts and ideas at present
accepted and the possibility of experimental verification. But other
explanations for the continuation of protein synthesis and morphogenesis in the absence of the nucleus are by no means ruled out. For
instance, Fox et al. (1962) recently discovered an interesting phenomenon
in Drosophila, which presents certain similarities with the Acetabularia
case. The presence of a Y-chromosome in the ovocyte results in the
291
In fact, the very presence of DNA is not necessary for morphogenesis
and protein synthesis, since these proceed at a normal rate in anucleate
fragments. On the other hand, RNA is present in large amounts in the
nucleolus and the nuclear sap of Acetabularia.
This nuclear RNA is
synthesized at a rapid rate and the experiments, at our present stage of
knowledge, indicate that it is transferred to the cytoplasm and accumulated in the tip of the alga, which is the site of morphogenesis. In other
words, there is a good deal of evidence, but no definite proof, for the
assumption that, in Acetabularia,
the 'morphogenetic substances'
of
Hämmerling (1953, 1959) could be equated to 'messenger'
{informational)
RNA. Such a simple hypothesis has an additional advantage: it easily
explains the cessation of regeneration and of protein synthesis 2 to 3
weeks after removal of the nucleus. If the Acetabularia nucleus produces
messenger RNA's, the latter must, in the anucleate cytoplasm, have a
relatively long life of 2 to 3 weeks (in contrast to the case of bacteria), or
be capable of independent synthesis in the absence of the nucleus. It is
conceivable that the supply of messenger RNA's in the anucleate
cytoplasm becomes exhausted after 2 to 3 weeks ; although the ribosomes
still exist at that time, their synthetic activity would cease because they
no longer receive messenger RNA's. A comparable situation has been
demonstrated recently in reticulocytes by Arnstein et al. (1962): the
ribosomes of these anucleate cells lose their activity when their
messenger RNA is removed.
It is obvious that the present hypothesis, that the morphogenetic
substances are messenger RNA molecules and that both morphogenesis
and protein synthesis stop, in the absence of the nucleus, when these
molecules have been exhausted, needs a good deal more experimental
support before we can accept it as a fact. Production by the nucleus of
messenger RNA, with a base composition similar to that of Acetabularia
DNA (which has not so far been isolated and analysed), must first be
demonstrated. One should also demonstrate the presence, in the
Acetabularia nucleus, of a DNA-primed RNA polymerase, which is
required to produce messenger RNA (Weiss and Nakamoto, 1961).
Finally, the lifetime of messenger RNA in the anucleate cytoplasm and
its accumulation at the tip of the alga still require experimental
demonstration. For the time being, the advantages of our hypothesis lie
in its simplicity, its agreement with known facts and ideas at present
accepted and the possibility of experimental verification. But other
explanations for the continuation of protein synthesis and morphogenesis in the absence of the nucleus are by no means ruled out. For
instance, Fox et al. (1962) recently discovered an interesting phenomenon
in Drosophila, which presents certain similarities with the Acetabularia
case. The presence of a Y-chromosome in the ovocyte results in the