272
JEAN BRÄCHET
and anucleate fragments of two different species. Their conclusion is
that the inhibition of morphogenesis by u.v. irradiation is due to the
destruction of morphogenetic substances endowed with species specificity, and they suggest that these substances might be cytoplasmic
RNA's. Such a conclusion is perfectly acceptable if we admit that this
cytoplasmic RNA is of nuclear origin and represents the so-called
'messenger' RNA.
Recent work by Werz (1960b, 1961) is in excellent agreement with the
experiments of Olszewska et ah (1961), which led us to the conclusion
that nuclear RNA accumulates at the apical end of the stalk, and that
its integrity is required for cap formation. Werz (1960b), using cytochemical methods for the detection of highly polymerized RNA, found
that this substance is strongly accumulated at the tip of the stalk, if
growth is normal. Keeping the cells in the dark reduces the RNA
content of the apex, whether the alga is nucleate or anucleate. If
anucleate fragments are kept for some time in the dark and then again
illuminated, the RNA content of the apical part increases. As pointed
out by Werz (1960b), there is a close correlation between this RNA and
the synthesis of the proteins that he detected in the same apical part of
the alga, by staining with azocarmine.
In more recent experiments, Werz (1961) succeeded in isolating
nuclei from Acetabularia algae crushed in 0· 1 % sucrose. Re-implantation
experiments showed that they 'survive' for 3 minutes. During that time,
the isolated nuclei eliminate vesicles rich in RNA and their RNA
content decreases. The conclusions of Werz (1961) are identical with
ours (Olszewska et al., 1961); there is a transfer of RNA from the
nucleus to the cytoplasm and this RNA of nuclear origin accumulates in
the apical part of the alga, where growth and morphogenesis occur.
The general conclusion is thus the same : there must be a very close
link, if not complete identity, between 'morphogenetic substances' and
nuclear (presumably 'messenger') RNA's.
3. RNA Synthesis in the Absence of the Nucleus
The possibility of independent net RNA synthesis in anucleate
fragments of Acetabularia was first reported by Brächet et al. (1955).
Despite the contradictory results of Richter (1959a, b) and Richter et ah
(1960), it appears that these results are valid, but that they are obtained
only under suitable experimental conditions (in particular, the intensity
of the light to which the algae are exposed). For instance, we found
(Naora et ah, 1960) that anucleate fragments of Acetabularia
readily
incorporate adenine, orotic acid and C0 2 into their cytoplasmic RNA;
net synthesis of RNA could be demonstrated, in the absence of the
nucleus, in the chloroplast fraction of the algae. The chloroplasts are
JEAN BRÄCHET
and anucleate fragments of two different species. Their conclusion is
that the inhibition of morphogenesis by u.v. irradiation is due to the
destruction of morphogenetic substances endowed with species specificity, and they suggest that these substances might be cytoplasmic
RNA's. Such a conclusion is perfectly acceptable if we admit that this
cytoplasmic RNA is of nuclear origin and represents the so-called
'messenger' RNA.
Recent work by Werz (1960b, 1961) is in excellent agreement with the
experiments of Olszewska et ah (1961), which led us to the conclusion
that nuclear RNA accumulates at the apical end of the stalk, and that
its integrity is required for cap formation. Werz (1960b), using cytochemical methods for the detection of highly polymerized RNA, found
that this substance is strongly accumulated at the tip of the stalk, if
growth is normal. Keeping the cells in the dark reduces the RNA
content of the apex, whether the alga is nucleate or anucleate. If
anucleate fragments are kept for some time in the dark and then again
illuminated, the RNA content of the apical part increases. As pointed
out by Werz (1960b), there is a close correlation between this RNA and
the synthesis of the proteins that he detected in the same apical part of
the alga, by staining with azocarmine.
In more recent experiments, Werz (1961) succeeded in isolating
nuclei from Acetabularia algae crushed in 0· 1 % sucrose. Re-implantation
experiments showed that they 'survive' for 3 minutes. During that time,
the isolated nuclei eliminate vesicles rich in RNA and their RNA
content decreases. The conclusions of Werz (1961) are identical with
ours (Olszewska et al., 1961); there is a transfer of RNA from the
nucleus to the cytoplasm and this RNA of nuclear origin accumulates in
the apical part of the alga, where growth and morphogenesis occur.
The general conclusion is thus the same : there must be a very close
link, if not complete identity, between 'morphogenetic substances' and
nuclear (presumably 'messenger') RNA's.
3. RNA Synthesis in the Absence of the Nucleus
The possibility of independent net RNA synthesis in anucleate
fragments of Acetabularia was first reported by Brächet et al. (1955).
Despite the contradictory results of Richter (1959a, b) and Richter et ah
(1960), it appears that these results are valid, but that they are obtained
only under suitable experimental conditions (in particular, the intensity
of the light to which the algae are exposed). For instance, we found
(Naora et ah, 1960) that anucleate fragments of Acetabularia
readily
incorporate adenine, orotic acid and C0 2 into their cytoplasmic RNA;
net synthesis of RNA could be demonstrated, in the absence of the
nucleus, in the chloroplast fraction of the algae. The chloroplasts are