NUCLEIC ACIDS AND SULPHYDRYL GROUPS
273
thus a major site of RNA synthesis in the cytoplasm of these algae. This
is in agreement with the recent observations of Schweiger and Bremer
(1961), who found that, if the algae are cultivated for some time in the
dark, cut, and then again given light, the anucleate fragments are the
site of net RNA synthesis. A net synthesis of RNA in the anucleate
fragments has also been observed by Sutter et al. (1961), who worked
with a different species of Acetabularia.
The evidence now available thus demonstrates a net synthesis of
chloroplastic RNA in anucleate fragments ; however, little information
is available concerning RNA synthesis in other fractions. Recent
experiments, still unpublished, by F. de Vitry strongly suggest that
soluble (transfer) RNA is synthesized, in the absence of the nucleus, in
the cytoplasm: the evidence is that 5-methylcytosine, an uncommon
base which is a part of transfer RNA, is incorporated in the soluble
RNA of anucleate fragments, even 3 weeks after removal of the nucleus.
This precursor behaves in an entirely different way from cytidine,
which accumulates first in the nucleus and then migrates to the
cytoplasm, as already found for other RNA precursors by Olszewska
et al. (1961). One cannot, however, exclude the fact that, in the
experiments of de Vitry, 5-methylcytosine is added as a terminal
nucleotide to pre-existing molecules of S-RNA. Further biochemical
work is obviously required before one can decide between the two
possibilities, terminal addition or complete synthesis. But one thing
seems certain: incorporation of 5-methylcytosine in S-RNA's of
anucleate fragments follows a very different pattern from that of
cytidine into high molecular weight RNA.
Ribosomes have also been studied by our co-workers Vanderhaeghe
and Baltus (1962). They found that 25 to 35% of the acid phosphatase
complement of the algae is bound to the ribosomes ; the balance is found
in the supernatant after ultra-centrifugation. When nucleate and
anucleate fragments are compared during a 2-week period, it is
observed that there is rapid acid phosphatase synthesis in the former,
but none whatsoever in the latter. The conclusion drawn above and
based on the work of Hämmerling et al. (1959), Keck and Clauss (1958)
and Keck (1960), that acid phosphatase is under a closer nuclear control
than the total proteins, can now be extended to the ribosomes : they are
no longer capable of synthesizing one of their own proteins in the
absence of the nucleus, a fact which suggests that production of the
ribosomes themselves might be closely controlled by the nucleus.
One would like to know something, of course, about the fate of
ribosomal RNA in anucleate fragments of Acetabularia.
Preliminary
experiments with
32
P have been performed by Vanderhaeghe and
Baltus on nucleate and anucleate fragments. They indicate, in good
273
thus a major site of RNA synthesis in the cytoplasm of these algae. This
is in agreement with the recent observations of Schweiger and Bremer
(1961), who found that, if the algae are cultivated for some time in the
dark, cut, and then again given light, the anucleate fragments are the
site of net RNA synthesis. A net synthesis of RNA in the anucleate
fragments has also been observed by Sutter et al. (1961), who worked
with a different species of Acetabularia.
The evidence now available thus demonstrates a net synthesis of
chloroplastic RNA in anucleate fragments ; however, little information
is available concerning RNA synthesis in other fractions. Recent
experiments, still unpublished, by F. de Vitry strongly suggest that
soluble (transfer) RNA is synthesized, in the absence of the nucleus, in
the cytoplasm: the evidence is that 5-methylcytosine, an uncommon
base which is a part of transfer RNA, is incorporated in the soluble
RNA of anucleate fragments, even 3 weeks after removal of the nucleus.
This precursor behaves in an entirely different way from cytidine,
which accumulates first in the nucleus and then migrates to the
cytoplasm, as already found for other RNA precursors by Olszewska
et al. (1961). One cannot, however, exclude the fact that, in the
experiments of de Vitry, 5-methylcytosine is added as a terminal
nucleotide to pre-existing molecules of S-RNA. Further biochemical
work is obviously required before one can decide between the two
possibilities, terminal addition or complete synthesis. But one thing
seems certain: incorporation of 5-methylcytosine in S-RNA's of
anucleate fragments follows a very different pattern from that of
cytidine into high molecular weight RNA.
Ribosomes have also been studied by our co-workers Vanderhaeghe
and Baltus (1962). They found that 25 to 35% of the acid phosphatase
complement of the algae is bound to the ribosomes ; the balance is found
in the supernatant after ultra-centrifugation. When nucleate and
anucleate fragments are compared during a 2-week period, it is
observed that there is rapid acid phosphatase synthesis in the former,
but none whatsoever in the latter. The conclusion drawn above and
based on the work of Hämmerling et al. (1959), Keck and Clauss (1958)
and Keck (1960), that acid phosphatase is under a closer nuclear control
than the total proteins, can now be extended to the ribosomes : they are
no longer capable of synthesizing one of their own proteins in the
absence of the nucleus, a fact which suggests that production of the
ribosomes themselves might be closely controlled by the nucleus.
One would like to know something, of course, about the fate of
ribosomal RNA in anucleate fragments of Acetabularia.
Preliminary
experiments with
32
P have been performed by Vanderhaeghe and
Baltus on nucleate and anucleate fragments. They indicate, in good