268
JEAN BRÄCHET
somes give spino-caudal inductions: the spino-caudal (mesodermal)
factor is thus presumably associated with the membrane component of
the endoplasmic reticulum.
However, even if the active inducing agents were pure proteins (basic
or not) there is no doubt that their synthesis has, at some stage of
development, been mediated by active ribosomes. In fact, recent
experiments by Toivonen et al. (1961) and by Yamada and Karasaki
(1962) have shown that substances which inhibit RNA synthesis
(ribonuclease, azaguanine, 5-fluoro-uracil) also inhibit neural induction.
Taken together, all the facts we have at hand indicate that ribonucleoproteins are actively inducing agents and that RNA synthesis is required
for induction in amphibian eggs.
B. Acetabularia
1. Protein Synthesis in the Absence of the Nucleus
It is now established beyond any doubt that proteins, including
several enzymes, are synthesized by Acetabularia in the absence of the
nucleus (Brächet et al., 1955). This demonstrates that the synthesis of
specific proteins—which are genetically controlled—is possible in the
absence of the genes themselves.
Hämmerling and his school have confirmed these observations
(Hämmerling et al., 1959), and analysed in more detail the protein
synthesis in nucleate and anucleate fragments, in which they found, for
instance, that protein synthesis occurs at the same rate (Clauss and
Werz, 1961). This observation is in contradiction with our earlier
finding that removal of the nucleus initially stimulated protein synthesis ;
our observations were probably due to the fact that, in these early
experiments, regeneration of the nucleate part was rather poor and
delayed. It is also of interest that anucleate fragments can synthesize
proteins even if they do not grow (Werz and Hämmerling, 1959) and
that protein synthesis is not increased when the cap forms or grows
(Clauss, 1962a). Finally, it was observed by Werz (1960a) that darkness
inhibits the synthesis of both total and chloroplastic proteins in nucleate
as well as in anucleate fragments. This is not surprising, since it is known
that darkness greatly reduces the size of the nucleus and of the nucleolus as well as the RNA content of the latter (Stich, 1951). It is likely
that the nucleus of the algae cultivated in darkness no longer functions
normally.
Protein synthesis in anucleate fragments ceases after a period of
about 3 weeks (Brächet et al., 1955). The reasons for the arrest of
protein synthesis remain unknown; recent work by Bremer et al. (1962)
shows that it is not due to a depletion of the total amino-acid pool ; but
one cannot yet exclude the possibility that the loss (partial or complete)
JEAN BRÄCHET
somes give spino-caudal inductions: the spino-caudal (mesodermal)
factor is thus presumably associated with the membrane component of
the endoplasmic reticulum.
However, even if the active inducing agents were pure proteins (basic
or not) there is no doubt that their synthesis has, at some stage of
development, been mediated by active ribosomes. In fact, recent
experiments by Toivonen et al. (1961) and by Yamada and Karasaki
(1962) have shown that substances which inhibit RNA synthesis
(ribonuclease, azaguanine, 5-fluoro-uracil) also inhibit neural induction.
Taken together, all the facts we have at hand indicate that ribonucleoproteins are actively inducing agents and that RNA synthesis is required
for induction in amphibian eggs.
B. Acetabularia
1. Protein Synthesis in the Absence of the Nucleus
It is now established beyond any doubt that proteins, including
several enzymes, are synthesized by Acetabularia in the absence of the
nucleus (Brächet et al., 1955). This demonstrates that the synthesis of
specific proteins—which are genetically controlled—is possible in the
absence of the genes themselves.
Hämmerling and his school have confirmed these observations
(Hämmerling et al., 1959), and analysed in more detail the protein
synthesis in nucleate and anucleate fragments, in which they found, for
instance, that protein synthesis occurs at the same rate (Clauss and
Werz, 1961). This observation is in contradiction with our earlier
finding that removal of the nucleus initially stimulated protein synthesis ;
our observations were probably due to the fact that, in these early
experiments, regeneration of the nucleate part was rather poor and
delayed. It is also of interest that anucleate fragments can synthesize
proteins even if they do not grow (Werz and Hämmerling, 1959) and
that protein synthesis is not increased when the cap forms or grows
(Clauss, 1962a). Finally, it was observed by Werz (1960a) that darkness
inhibits the synthesis of both total and chloroplastic proteins in nucleate
as well as in anucleate fragments. This is not surprising, since it is known
that darkness greatly reduces the size of the nucleus and of the nucleolus as well as the RNA content of the latter (Stich, 1951). It is likely
that the nucleus of the algae cultivated in darkness no longer functions
normally.
Protein synthesis in anucleate fragments ceases after a period of
about 3 weeks (Brächet et al., 1955). The reasons for the arrest of
protein synthesis remain unknown; recent work by Bremer et al. (1962)
shows that it is not due to a depletion of the total amino-acid pool ; but
one cannot yet exclude the possibility that the loss (partial or complete)