84
EDWARD C. CANTINO AND JAMES S. LOVETT
the insoluble nitrogen (see preceding paragraphs), together with the
evidence that chromospheres participate in the formation of nuclear caps
in Allomyces, led to the suggestion (Cantino, 1961b) that this NaCl
insoluble RNA is laid down during differentiation of RS cells in
Blastocladiella in the form of protein-bound organelles, perhaps similar
to chromospheres. If so, presumably these would be incorporated—or
transformed—into nuclear caps at the time zoospores are formed. This
idea has now been greatly strengthened with the finding that, whereas
the NaCl-insoluble RNA is ca. 65% of the total RNA in a mature RS cell,
the RNA in nuclear caps isolated from the spores themselves is 68%
of the total RNA in the spore (Lovett, 1963).
What likely explanations might be offered for the way in which these
organelles are formed during differentiation? Perhaps pre-existing
ribosomal material, present at the end of exponential growth, aggregates
directly to form the hypothetical intermediates, the chromosphere-like
organelles. This would mean, of course, that the observed alteration of
base ratios occurs via synthesis of a special RNA with unusual nucleotide
composition during differentiation; note that 35% of the total RNA in
the cell is synthesized during this period. Alternatively, all of the NaClinsoluble RNA may be newly synthesized, in part from turnover of the
pre-existing NaCl-soluble RNA which decreases during this interval in
ontogeny, and in part by way of synthesis from other raw ingredients.
The second explanation is favoured at present, and allows for alteration
of base ratios during the process of resynthesis. In either case, differences
in solubility may be due to the physical state of the RNA in the newlyformed aggregates. The increase in activity of the glucose-6-phosphate
dehydrogenase during maturation of the RS cell—the only enzyme
activity known to increase at this time—suggests further that the
hexose monophosphate shunt may operate to provide both the additional
supply of ribose and the energy needed for continued synthesis of RNA.
Furthermore, the required raw material may well be the abundant pool
of polysaccharide, which decreases sharply during RS differentiation.
Chromospheres have not been detected cytologically in
Blastocladiella
however, and for this reason it is obvious that a large measure of
speculation is involved. Nevertheless, this notion is consistent with all
the other known facts about the process. A complete and unequivocal
interpretation of the changes which transpire during the origin and
maturation of an RS cell must now await a careful analysis by a
combination of cell fractionations and electron microscopy. The value
of this approach has already been amply demonstrated by studies of the
motile spore of Blastocladiella (Lovett, 1963), not to mention investigations of other biological systems.
After some 84 hours, when all the aforementioned machinations of the
EDWARD C. CANTINO AND JAMES S. LOVETT
the insoluble nitrogen (see preceding paragraphs), together with the
evidence that chromospheres participate in the formation of nuclear caps
in Allomyces, led to the suggestion (Cantino, 1961b) that this NaCl
insoluble RNA is laid down during differentiation of RS cells in
Blastocladiella in the form of protein-bound organelles, perhaps similar
to chromospheres. If so, presumably these would be incorporated—or
transformed—into nuclear caps at the time zoospores are formed. This
idea has now been greatly strengthened with the finding that, whereas
the NaCl-insoluble RNA is ca. 65% of the total RNA in a mature RS cell,
the RNA in nuclear caps isolated from the spores themselves is 68%
of the total RNA in the spore (Lovett, 1963).
What likely explanations might be offered for the way in which these
organelles are formed during differentiation? Perhaps pre-existing
ribosomal material, present at the end of exponential growth, aggregates
directly to form the hypothetical intermediates, the chromosphere-like
organelles. This would mean, of course, that the observed alteration of
base ratios occurs via synthesis of a special RNA with unusual nucleotide
composition during differentiation; note that 35% of the total RNA in
the cell is synthesized during this period. Alternatively, all of the NaClinsoluble RNA may be newly synthesized, in part from turnover of the
pre-existing NaCl-soluble RNA which decreases during this interval in
ontogeny, and in part by way of synthesis from other raw ingredients.
The second explanation is favoured at present, and allows for alteration
of base ratios during the process of resynthesis. In either case, differences
in solubility may be due to the physical state of the RNA in the newlyformed aggregates. The increase in activity of the glucose-6-phosphate
dehydrogenase during maturation of the RS cell—the only enzyme
activity known to increase at this time—suggests further that the
hexose monophosphate shunt may operate to provide both the additional
supply of ribose and the energy needed for continued synthesis of RNA.
Furthermore, the required raw material may well be the abundant pool
of polysaccharide, which decreases sharply during RS differentiation.
Chromospheres have not been detected cytologically in
Blastocladiella
however, and for this reason it is obvious that a large measure of
speculation is involved. Nevertheless, this notion is consistent with all
the other known facts about the process. A complete and unequivocal
interpretation of the changes which transpire during the origin and
maturation of an RS cell must now await a careful analysis by a
combination of cell fractionations and electron microscopy. The value
of this approach has already been amply demonstrated by studies of the
motile spore of Blastocladiella (Lovett, 1963), not to mention investigations of other biological systems.
After some 84 hours, when all the aforementioned machinations of the