274
JEAN BRÄCHET
agreement with earlier findings, that removal of the nucleus inhibits
ribosomal RNA synthesis to a much greater extent than that of
chloroplastic RNA. However, according to Sutter et al. (1961), strong
incorporation of adenine into ribosomal RNA can be observed in
anucleate fragments of the American species of Acetabularia
used in
their experiments. For the present, this question thus remains unsettled.
It is further confused by the fact that, according to observations of
Vanderhaeghe with the electron microscope, ribosomes are present in
the chloroplasts. Whether the RNA metabolism of these chloroplastic
ribosomes is different, in the absence of the nucleus, from that of free
ribosomes, is still another problem for the future.
Pulse experiments with
3 2
P have also been performed in our laboratory
by M. Ceska (1962): they demonstrated the complexity of a situation
which deserves a detailed analysis. Using pulses of 5 min, 15 min, and
24 h, he detected four RNA peaks with different base compositions. In
the longer (24 h) pulse experiments, a single high molecular species of
RNA was labelled. In anucleate fragments, 1 day after section, the
three RNA components of low molecular weight are still synthesized ;
but the synthesis of the high molecular weight RNA is suppressed. It
will, of course, be important to repeat these experiments on algae which
have been cut for a longer time : after 1 day, the fragments are still
recovering from the operative shock.
It is much too early to draw any conclusions, since the whole subject
of RNA synthesis in anucleate fragments of Acetabularia remains in a
fluid state. A recent observation of Vanderhaeghe and Baltus brings a
further warning : they found that cap formation inhibits acid phosphatase
synthesis in nucleate as well as in anucleate fragments and that the
ribosomal enzyme undergoes synthesis, in the absence of the nucleus,
when the enucleation experiments are performed on young growing
algae. This observation is in agreement with the above mentioned
results of Clauss (1962a), that protein synthesis is not increased when
caps form and grow. The experiments which have been done in the past
retain their full value for the problem in which we are interested here :
the biochemical mechanisms of nuclear control on morphogenesis.
But
they will have to be repeated on young, actively growing algae if we are
to understand better how the nucleus controls the overall biochemical
activities of the cell.
IV. Sulphydryl Groups and Morphogenesis
A. Introductory Remarks
The importance of sulphydryl (thiol or —SH) groups for morphogenesis has often been emphasized : this fact has been clearly shown, for
JEAN BRÄCHET
agreement with earlier findings, that removal of the nucleus inhibits
ribosomal RNA synthesis to a much greater extent than that of
chloroplastic RNA. However, according to Sutter et al. (1961), strong
incorporation of adenine into ribosomal RNA can be observed in
anucleate fragments of the American species of Acetabularia
used in
their experiments. For the present, this question thus remains unsettled.
It is further confused by the fact that, according to observations of
Vanderhaeghe with the electron microscope, ribosomes are present in
the chloroplasts. Whether the RNA metabolism of these chloroplastic
ribosomes is different, in the absence of the nucleus, from that of free
ribosomes, is still another problem for the future.
Pulse experiments with
3 2
P have also been performed in our laboratory
by M. Ceska (1962): they demonstrated the complexity of a situation
which deserves a detailed analysis. Using pulses of 5 min, 15 min, and
24 h, he detected four RNA peaks with different base compositions. In
the longer (24 h) pulse experiments, a single high molecular species of
RNA was labelled. In anucleate fragments, 1 day after section, the
three RNA components of low molecular weight are still synthesized ;
but the synthesis of the high molecular weight RNA is suppressed. It
will, of course, be important to repeat these experiments on algae which
have been cut for a longer time : after 1 day, the fragments are still
recovering from the operative shock.
It is much too early to draw any conclusions, since the whole subject
of RNA synthesis in anucleate fragments of Acetabularia remains in a
fluid state. A recent observation of Vanderhaeghe and Baltus brings a
further warning : they found that cap formation inhibits acid phosphatase
synthesis in nucleate as well as in anucleate fragments and that the
ribosomal enzyme undergoes synthesis, in the absence of the nucleus,
when the enucleation experiments are performed on young growing
algae. This observation is in agreement with the above mentioned
results of Clauss (1962a), that protein synthesis is not increased when
caps form and grow. The experiments which have been done in the past
retain their full value for the problem in which we are interested here :
the biochemical mechanisms of nuclear control on morphogenesis.
But
they will have to be repeated on young, actively growing algae if we are
to understand better how the nucleus controls the overall biochemical
activities of the cell.
IV. Sulphydryl Groups and Morphogenesis
A. Introductory Remarks
The importance of sulphydryl (thiol or —SH) groups for morphogenesis has often been emphasized : this fact has been clearly shown, for