32
J. B. GURDON
cell type to another, perhaps according to how soon after their formation they are involved in promoting synthesis. They would be detected
in Acetabularia because they happen to accumulate in the dark.
Further information on persistent cytoplasmic substances capable of
promoting protein synthesis has been offered by Keck (1961). He found
that Acetabularia mediterranea and Acicularia schenckii have different
alkaline phosphatase enzymes distinguishable electrophoretically. An
anucleate Acicularia continues to synthesize its own kind of enzyme for
several days, but if an Acetabularia rhizoid is grafted to it, the enzyme
synthesized changes in the next few days to the Acetabularia kind. The
reciprocal experiment does not yield the same result, since an anucleate
Acetabularia continues to synthesize its own kind of enzyme whether an
Acicularia rhizoid is grafted to it or not. Keck interprets these results
as illustrating a kind of molecular conversion of proteins in the cytoplasm. However they could equally well be explained by supposing that
Acetabularia cytoplasm destroys the Acicularia enzyme. These results
do not necessarily show that the morphogenetic substances which
persist in the cytoplasm can be specifically altered by exposure to a
foreign rhizoid. They do however confirm the existence of these substances in Acetabularia.
The reader is referred to a recent review by Hämmerling (1963).
2. Movement of labelled substances in Amoeba
The passage of RNA and protein between nucleus and cytoplasm in
Amoeba has been followed by transplanting labelled nuclei to unlabelled
cytoplasm and vice versa (Goldstein andPlaut, 1955; Goldstein, 1963a).
The technique has been outlined in Section II, A.
The movement of RNA has been followed in this way. A nucleus from
an unlabelled amoeba was transplanted to an enucleated amoeba which
had been in tritiated adenosine for 1-4 h, and as a control a labelled
nucleus was transplanted to an enucleated unlabelled amoeba. Both
kinds of amoebae were then incubated for 2-8 h in medium containing
a lot of unlabelled 'chaser' adenosine. When the amoebae had been
fixed a few hours later, it was found that the amoeba with the labelled
nucleus had much more radioactive RNA in its cytoplasm than the
one with the unlabelled nucleus. The RNA was synthesized in the
nucleus, and it is almost certain that it was not rapidly broken down
in the nucleus and resynthesized again in the cytoplasm. The passage
of RNA from nucleus to cytoplasm demonstrated by this work is
entirely consistent with the current genetical and biochemical evidence
for a messenger RNA as well as with the work on Acetabularia discussed
above.
Of a unique nature is the evidence for the movement of a protein
J. B. GURDON
cell type to another, perhaps according to how soon after their formation they are involved in promoting synthesis. They would be detected
in Acetabularia because they happen to accumulate in the dark.
Further information on persistent cytoplasmic substances capable of
promoting protein synthesis has been offered by Keck (1961). He found
that Acetabularia mediterranea and Acicularia schenckii have different
alkaline phosphatase enzymes distinguishable electrophoretically. An
anucleate Acicularia continues to synthesize its own kind of enzyme for
several days, but if an Acetabularia rhizoid is grafted to it, the enzyme
synthesized changes in the next few days to the Acetabularia kind. The
reciprocal experiment does not yield the same result, since an anucleate
Acetabularia continues to synthesize its own kind of enzyme whether an
Acicularia rhizoid is grafted to it or not. Keck interprets these results
as illustrating a kind of molecular conversion of proteins in the cytoplasm. However they could equally well be explained by supposing that
Acetabularia cytoplasm destroys the Acicularia enzyme. These results
do not necessarily show that the morphogenetic substances which
persist in the cytoplasm can be specifically altered by exposure to a
foreign rhizoid. They do however confirm the existence of these substances in Acetabularia.
The reader is referred to a recent review by Hämmerling (1963).
2. Movement of labelled substances in Amoeba
The passage of RNA and protein between nucleus and cytoplasm in
Amoeba has been followed by transplanting labelled nuclei to unlabelled
cytoplasm and vice versa (Goldstein andPlaut, 1955; Goldstein, 1963a).
The technique has been outlined in Section II, A.
The movement of RNA has been followed in this way. A nucleus from
an unlabelled amoeba was transplanted to an enucleated amoeba which
had been in tritiated adenosine for 1-4 h, and as a control a labelled
nucleus was transplanted to an enucleated unlabelled amoeba. Both
kinds of amoebae were then incubated for 2-8 h in medium containing
a lot of unlabelled 'chaser' adenosine. When the amoebae had been
fixed a few hours later, it was found that the amoeba with the labelled
nucleus had much more radioactive RNA in its cytoplasm than the
one with the unlabelled nucleus. The RNA was synthesized in the
nucleus, and it is almost certain that it was not rapidly broken down
in the nucleus and resynthesized again in the cytoplasm. The passage
of RNA from nucleus to cytoplasm demonstrated by this work is
entirely consistent with the current genetical and biochemical evidence
for a messenger RNA as well as with the work on Acetabularia discussed
above.
Of a unique nature is the evidence for the movement of a protein