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Plant Physiol, November 1999, Vol. 121, pp. 965-975
The Allosterically Unregulated Isoform of ADP-Glucose
Pyrophosphorylase from Barley Endosperm Is the Most Likely Source of
ADP-Glucose Incorporated into Endosperm Starch1
Danny N.P.
Doan,2
Heidi
Rudi, and
Odd-Arne
Olsen*
Plant Molecular Biology Laboratory, Agricultural University of
Norway, P.O. Box 5051, N-1432 Ås, Norway
We
present the results of studies of an unmodified version of the
recombinant major barley (Hordeum vulgare) endosperm
ADP-glucose pyrophoshorylase (AGPase) expressed in insect cells, which
corroborate previous data that this isoform of the enzyme acts
independently of the allosteric regulators 3-phosphoglycerate and
inorganic phosphate. We also present a characterization of the
individual subunits expressed separately in insect cells, showing that
the SS AGPase is active in the presence of 3-phosphoglycerate and is
inhibited by inorganic phosphate. As a step toward the elucidation of
the role of the two AGPase isoforms in barley, the temporal and spatial
expression profile of the four barley AGPase transcripts encoding these
isoforms were studied. The results show that the steady-state level of
beps and bepl, the transcripts encoding the major endosperm isoform, correlated positively with the rate of
endosperm starch accumulation. In contrast, blps and
blpl, the transcripts encoding the major leaf isoform,
were constitutively expressed at a very low steady-state level
throughout the barley plant. The implications of these findings for the
evolution of plant AGPases are discussed.
1
This work has been funded by the European Union
project "Genetic Tailoring of Novel Starch Polymers-CT95-0568" and
the Biotechnology Program of the Norwegian Research Council.
2
Present address: Institute of Molecular
Agrobiology, 1 Research Link, National University of Singapore,
Singapore 117604.
*
Corresponding author; e-mail odd-arne.olsen{at}ikb.nlh.no; fax
47-64941465.
© 1999 American Society of Plant Physiologists
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