Plant Molecular Biology 33: 431–444, 1997.
1997 Kluwer Academic Publishers. Printed in Belgium.
Molecular cloning and characterization of ADP-glucose pyrophosphorylase
cDNA clones isolated from pea cotyledons
, Andrea Penton
, Pamela Dunsmuir and Hugo Dooner
DNA Plant Technology Corporation, 6701 San Pablo Ave., Oakland, CA 94608, USA (
present address: McArdle Laboratories, 1400 University Ave., Madison, WI 53706, USA;
present address: The Waksman Institute, Rutgers University, Piscataway, NJ 08855, USA
Received 29 May 1996; accepted in revised form 25 September 1996
Key words: ADP-glucose pyrophosphorylase, cDNA clones, Pisum sativum, starch biosynthesis
Three ADP-glucose pyrophosphorylase (ADPG-PPase) cDNA clones have been isolated and characterized from a
pea cotyledon cDNA library. Two of these clones (Psagps1 and Psagps2) encode the small subunit of ADPG-PPase.
The deduced amino acid sequences for these two clones are 95% identical. Expression of these two genes differs
in that the Psagps2 gene shows comparatively higher expression in seeds relative to its expression in other tissues.
Psagps2 expression also peaks midway through seed development at a time in which Psagps1 transcripts are still
accumulating. The third cDNA isolated (Psagpl1) encodes the large subunit of ADPG-PPase. It shows greater
selectivity in expression than either of the small subunit clones. It is highly expressed in sink organs (seed, pod,
and seed coat) and undetectable in leaves.
ADP-glucose pyrophosphorylase (ADPG-PPase; EC
18.104.22.168) catalyzes the ﬁrst committed step in starch
biosynthesis in which the activated glucosyl donor,
ADP-glucose, is formed from glucose-1-P plus ATP.
In the plastid this reaction is made irreversible by
an alkaline inorganic pyrophosphatase which rapidly
hydrolyzes the second product of this reaction, inor-
ganic pyrophosphate. ADP-glucose then serves as
a substrate for starch synthase which adds the glucosyl
moiety of ADP-glucose to the non-reducing end of an
(1-4) glucan primer.
In the chloroplasts of photosynthetic cells ADPG-
PPase is an allosterically-regulated enzyme, being
activated by 3-phosphoglycerate(3PGA) and inhibited
by inorganic phosphate (P
) . This allows starch
synthesis to be responsive to the amount of photosyn-
thate (3PGA) and ATP present in the chloroplast, as
The nucleotide sequence data reported will appear in the
EMBL, GenBank, and DDBJ Nucleotide Sequence Databases under
the accession numbers X96764 (Psagps1), X96765 (Psagps2)and
well as to the rate of photosynthate export. ATP syn-
thesis requires P
also functions in the export
of triose phosphates from the chloroplast via the phos-
phate translocator. In storage organs it is less clear
how important allosteric regulation of ADPG-PPase is
in controlling starch biosynthesis. Reduced carbon is
imported into amyloplasts not as triose phosphates but
as hexose phosphates  or possibly even as ADP-
glucose . It has recently been shown that at least
in cereal endosperms a cytosolic form of ADPG-PPase
exists in addition to the amyloplastic form . While
the isoform from maize endosperm is as sensitive to
modulators as the leaf isoform , ADPG-PPases
from pea and Vicia faba embryos as well as wheat and
barley endosperm show much less sensitivity to mod-
ulators than their leaf counterparts [19, 24, 35, 57]. In
addition, at least in pea embryos, ADPG-PPase does
not play as important a role in controlling the rate of
starch biosynthesis as it does in leaves .
While bacterial ADPG-PPase is a homotetrameric
enzyme, in plants it is a heterotetramer composed of
two different but evolutionarily related subunit types
. These subunit types are referred to as the large