296
JEAN BRÄCHET
have relatively normal nuclei. In a third class of nuclei, which contain
an excess of RNA (vide supra), messenger RNA's, presumably abnormal
in composition, might be formed which cannot leave the DNA templates.
In 1954, we suggested that, in lethal hybrids, nuclear RNA, which
might be abnormal in composition, cannot leave the nucleus and be
utilized by the cytoplasm. This conclusion was shared by Zeller (1956),
who worked on other lethal combinations. We have also seen that, in
our recent experiments with sea urchin hybrids, autoradiography shows
that the migration of nuclear RNA towards the cytoplasm is hindered
(Ficq and Brächet, unpublished). There is a counterpart in bacteria of
our findings on lethal hybrids: according to Otaka et al. (1961), treatment of Bacillus cereus with azaguanine leads to the accumulation and
trapping of an abnormal messenger RNA. This is exactly the situation
we believe to exist in lethal hybrids.
In the hybrids, the ribosomes might also remain inhibited or repressed, so that they could not, in any event, accept messenger RNA's.
Grafting of a piece of the lethal hybrid in a normal host is followed by
'revitalization'. It is difficult to explain the latter by the diffusion of host
messenger RNA and its fixation on the hybrid ribosomes, since there is
no species-specificity in the revitalization process. It is easier to imagine
that the normal host somehow suppresses, in a non-specific way, the
inhibition of the ribosomes: messenger RNA could now more easily
leave its DNA template and reach the ribosomal template. It certainly
could be of great interest to know whether revitalization is still possible
under conditions where RNA synthesis is inhibited.
There is no doubt that the ideas which have been presented in this
discussion are highly speculative and they may well prove to be entirely
wrong. They have the advantage that many of them can be submitted
to experimental test. But it seems useless to try to speculate further,
since what we really need is more facts. In any event, it was amusing to
try to explain the phenomenon of differentiation in the terms of
molecular biology and to find that, in differentiating systems as elsewhere, genetic information is apparently found in the nucleic acids only,
while the phenotypic expression of genetic activity seems to lie in the
fine structure of the proteins synthesized under nuclear control.
Acknowledgements
The experiments from this laboratory presented in this review were
made possible thanks to the financial aid of Euratom (Contrat 016-61-10
ABIB) and the EOARDC (Contrat AF 61(052)-356). We wish to
express our warmest thanks to these two organizations.
Our warmest thanks are also due to Dr. Ann Roller, who kindly
accepted the task of improving the English in the present text.
JEAN BRÄCHET
have relatively normal nuclei. In a third class of nuclei, which contain
an excess of RNA (vide supra), messenger RNA's, presumably abnormal
in composition, might be formed which cannot leave the DNA templates.
In 1954, we suggested that, in lethal hybrids, nuclear RNA, which
might be abnormal in composition, cannot leave the nucleus and be
utilized by the cytoplasm. This conclusion was shared by Zeller (1956),
who worked on other lethal combinations. We have also seen that, in
our recent experiments with sea urchin hybrids, autoradiography shows
that the migration of nuclear RNA towards the cytoplasm is hindered
(Ficq and Brächet, unpublished). There is a counterpart in bacteria of
our findings on lethal hybrids: according to Otaka et al. (1961), treatment of Bacillus cereus with azaguanine leads to the accumulation and
trapping of an abnormal messenger RNA. This is exactly the situation
we believe to exist in lethal hybrids.
In the hybrids, the ribosomes might also remain inhibited or repressed, so that they could not, in any event, accept messenger RNA's.
Grafting of a piece of the lethal hybrid in a normal host is followed by
'revitalization'. It is difficult to explain the latter by the diffusion of host
messenger RNA and its fixation on the hybrid ribosomes, since there is
no species-specificity in the revitalization process. It is easier to imagine
that the normal host somehow suppresses, in a non-specific way, the
inhibition of the ribosomes: messenger RNA could now more easily
leave its DNA template and reach the ribosomal template. It certainly
could be of great interest to know whether revitalization is still possible
under conditions where RNA synthesis is inhibited.
There is no doubt that the ideas which have been presented in this
discussion are highly speculative and they may well prove to be entirely
wrong. They have the advantage that many of them can be submitted
to experimental test. But it seems useless to try to speculate further,
since what we really need is more facts. In any event, it was amusing to
try to explain the phenomenon of differentiation in the terms of
molecular biology and to find that, in differentiating systems as elsewhere, genetic information is apparently found in the nucleic acids only,
while the phenotypic expression of genetic activity seems to lie in the
fine structure of the proteins synthesized under nuclear control.
Acknowledgements
The experiments from this laboratory presented in this review were
made possible thanks to the financial aid of Euratom (Contrat 016-61-10
ABIB) and the EOARDC (Contrat AF 61(052)-356). We wish to
express our warmest thanks to these two organizations.
Our warmest thanks are also due to Dr. Ann Roller, who kindly
accepted the task of improving the English in the present text.