NUCLEIC ACIDS AND SULPHYDRYL GROUPS
295
are inactive (or repressed) and a great number of substances can activate
them (i.e., de-repress them).
According to this picture, the ribosomes of unfertilized eggs and
amphibian morulae would be strikingly similar to those of bacteria
treated with certain antibiotics: inhibition of protein synthesis, by
streptomycin, according to a hypothesis of Potts and Stanier (1961), is
due to the binding of this antibiotic to the ribosomes in the streptomycinsensitive strains. This hypothesis has recently been confirmed by Flaks
et al. (1962): they demonstrated that combination of one or two
molecules of streptomycin to each ribosome is sufficient to block the
access of messenger RNA to the ribosomes. A normally occurring basic
protein (Crampton and Petermann, 1959; Spahr, 1962) or peptide might
play the same role as streptomycin in bacteria ; its production at the end
of ovogenesis might block the synthetic activity of the ribosomes. At the
onset of gastrulation, this block is lifted and messenger RNA's which are
now being produced by the nuclei, can reach the ribosomal templates.
To explain differentiation at later stages, the best hypothesis, for the
time being, seems to be that of a competition between ribosomes which,
after removal of the inhibitor, have become loaded with different
messenger RNA's. This is an idea which has been often defended when
the plasma gene theory was fashionable. Many different specific
messenger RNA molecules are certainly produced by the nuclei; but
only one (or certain) of them would be accepted, at a given stage of
development, by a given kind of ribosomes. The end-result would be the
formation of haemoglobin (and red blood cells) in certain areas of the
embryo, and of actomyosin (and muscle cells) in others. Recent work by
Scholtissek (1962) brings some experimental support for such a view:
his experiments on nuclei and cytoplasmic fractions isolated from
various organs indicate that the cytoplasm of each organ selects and
stabilizes the various RNA species presented by the nuclei. For instance,
kidney cytoplasm selects another kind of nuclear RNA than liver
cytoplasm in this in vitro system. If nuclear RNA cannot attach itself to
a ribosomal template, quick destruction follows.
The hypothesis which has just been presented is that of a progressive
'de-repression' of ribosomes, which would become capable of accepting
specific RNA's and competing successfully with other ribosomes. Such a
hypothesis can be extended to the case of lethal hybrids in which
development is normal during cleavage. Here, as we have recently
suggested (Brächet et al., 1962),
e
hybrid' DNA molecules could form at
the blastula stage, when the two genomes, paternal and maternal, mix.
In the hybrids, formation of hybrid DNA molecules might occur at
random. In proportion to the frequency of illegitimate pairing, some
nuclei would be produced which form no RNA at all, while others would
M2
295
are inactive (or repressed) and a great number of substances can activate
them (i.e., de-repress them).
According to this picture, the ribosomes of unfertilized eggs and
amphibian morulae would be strikingly similar to those of bacteria
treated with certain antibiotics: inhibition of protein synthesis, by
streptomycin, according to a hypothesis of Potts and Stanier (1961), is
due to the binding of this antibiotic to the ribosomes in the streptomycinsensitive strains. This hypothesis has recently been confirmed by Flaks
et al. (1962): they demonstrated that combination of one or two
molecules of streptomycin to each ribosome is sufficient to block the
access of messenger RNA to the ribosomes. A normally occurring basic
protein (Crampton and Petermann, 1959; Spahr, 1962) or peptide might
play the same role as streptomycin in bacteria ; its production at the end
of ovogenesis might block the synthetic activity of the ribosomes. At the
onset of gastrulation, this block is lifted and messenger RNA's which are
now being produced by the nuclei, can reach the ribosomal templates.
To explain differentiation at later stages, the best hypothesis, for the
time being, seems to be that of a competition between ribosomes which,
after removal of the inhibitor, have become loaded with different
messenger RNA's. This is an idea which has been often defended when
the plasma gene theory was fashionable. Many different specific
messenger RNA molecules are certainly produced by the nuclei; but
only one (or certain) of them would be accepted, at a given stage of
development, by a given kind of ribosomes. The end-result would be the
formation of haemoglobin (and red blood cells) in certain areas of the
embryo, and of actomyosin (and muscle cells) in others. Recent work by
Scholtissek (1962) brings some experimental support for such a view:
his experiments on nuclei and cytoplasmic fractions isolated from
various organs indicate that the cytoplasm of each organ selects and
stabilizes the various RNA species presented by the nuclei. For instance,
kidney cytoplasm selects another kind of nuclear RNA than liver
cytoplasm in this in vitro system. If nuclear RNA cannot attach itself to
a ribosomal template, quick destruction follows.
The hypothesis which has just been presented is that of a progressive
'de-repression' of ribosomes, which would become capable of accepting
specific RNA's and competing successfully with other ribosomes. Such a
hypothesis can be extended to the case of lethal hybrids in which
development is normal during cleavage. Here, as we have recently
suggested (Brächet et al., 1962),
e
hybrid' DNA molecules could form at
the blastula stage, when the two genomes, paternal and maternal, mix.
In the hybrids, formation of hybrid DNA molecules might occur at
random. In proportion to the frequency of illegitimate pairing, some
nuclei would be produced which form no RNA at all, while others would
M2