328
J . S E E D
metabolically stable, a fraction of the nuclear R N A turns over and in
doing so some of the precursors are re-incorporated into D N A (Feinendegen, Bond and Painter, 1961c; Feinendegen, Bond and Hughes,
1961b). In addition, unlabelled thymidine was found to inhibit partially
the uptake of tritiated cytidine into R N A , but excess cytidine had little
effect on the incorporation of tritiated thymidine into D N A (Feinendegen et al., 1961b).
Another approach was used by Perry, Hell and Errera (1961) who,
working with individual HeLa cells, irradiated the nucleoli and chromatin separately with an u.v. microbeam and observed the effects on
cytidine uptake by means of autoradiography. From their results they deduced that, in HeLa cells, two-thirds ofthe label eventually incorporated
into cytoplasmic R N A was dependent on the nucleolus, but that less
than one-third ofthe uptake into the chromatin was similarly dependent.
The earlier work of Seed (1960) had shown that, in fresh cultures of
mouse fibroblasts, X-irradiation of the nucleolus in the pre-synthetic
period resulted, several hours later, in a reduction of total nuclear
nucleic acid relative to the controls: a similar X-irradiation of the
nuclear sap, i.e. avoiding the nucleoli, did not cause this effect. In this
investigation the amounts of nucleic acid in each cell nucleus were determined from u.v. absorption measurements.
An interpretation different from that advanced by Marshak and
others for the experiments on the incorporation of nucleosides has been
put forward by Harris (1959b, 1960, 1962). Harris has pointed out that
the kinetics, explained by some authors in terms of nuclear-RNA
synthesis followed by migration to the cytoplasm, might in many cases
be explained on the basis that both nuclear and cytoplasmic R N A are
derived from acid-soluble precursors, the cytoplasmic formation being
the slower process. In most of the autoradiographic experiments
claimed to support the migration of nuclear R N A , little or no attempt
has been made to correlate the amount of nuclear label lost with the
amount gained by the cytoplasm.
Using a non-dividing cell in culture, the rabbit macrophage, Watts
and Harris (1959) detected a rapid turnover of R N A by the incorporation and subsequent release of
1 4
C adenine: on the other hand, in
experiments using
3 2
P label, the incorporated label could not be displaced by unlabelled orthophosphate over similar time periods up
to 12 h. This observation might be explained by the turnover of the
terminal adenylic acid of transfer R N A (see, for example, Vincent and
Baltus, 1960; Hoagland, 1960).
However, most of the experiments favouring a migration of R N A
from nucleus to cytoplasm have used rapidly-dividing cells, in which
there is an appreciable net R N A synthesis appropriate to the increase
J . S E E D
metabolically stable, a fraction of the nuclear R N A turns over and in
doing so some of the precursors are re-incorporated into D N A (Feinendegen, Bond and Painter, 1961c; Feinendegen, Bond and Hughes,
1961b). In addition, unlabelled thymidine was found to inhibit partially
the uptake of tritiated cytidine into R N A , but excess cytidine had little
effect on the incorporation of tritiated thymidine into D N A (Feinendegen et al., 1961b).
Another approach was used by Perry, Hell and Errera (1961) who,
working with individual HeLa cells, irradiated the nucleoli and chromatin separately with an u.v. microbeam and observed the effects on
cytidine uptake by means of autoradiography. From their results they deduced that, in HeLa cells, two-thirds ofthe label eventually incorporated
into cytoplasmic R N A was dependent on the nucleolus, but that less
than one-third ofthe uptake into the chromatin was similarly dependent.
The earlier work of Seed (1960) had shown that, in fresh cultures of
mouse fibroblasts, X-irradiation of the nucleolus in the pre-synthetic
period resulted, several hours later, in a reduction of total nuclear
nucleic acid relative to the controls: a similar X-irradiation of the
nuclear sap, i.e. avoiding the nucleoli, did not cause this effect. In this
investigation the amounts of nucleic acid in each cell nucleus were determined from u.v. absorption measurements.
An interpretation different from that advanced by Marshak and
others for the experiments on the incorporation of nucleosides has been
put forward by Harris (1959b, 1960, 1962). Harris has pointed out that
the kinetics, explained by some authors in terms of nuclear-RNA
synthesis followed by migration to the cytoplasm, might in many cases
be explained on the basis that both nuclear and cytoplasmic R N A are
derived from acid-soluble precursors, the cytoplasmic formation being
the slower process. In most of the autoradiographic experiments
claimed to support the migration of nuclear R N A , little or no attempt
has been made to correlate the amount of nuclear label lost with the
amount gained by the cytoplasm.
Using a non-dividing cell in culture, the rabbit macrophage, Watts
and Harris (1959) detected a rapid turnover of R N A by the incorporation and subsequent release of
1 4
C adenine: on the other hand, in
experiments using
3 2
P label, the incorporated label could not be displaced by unlabelled orthophosphate over similar time periods up
to 12 h. This observation might be explained by the turnover of the
terminal adenylic acid of transfer R N A (see, for example, Vincent and
Baltus, 1960; Hoagland, 1960).
However, most of the experiments favouring a migration of R N A
from nucleus to cytoplasm have used rapidly-dividing cells, in which
there is an appreciable net R N A synthesis appropriate to the increase