Z. Sun et al., DIFFERENTIAL EXPRESSION OF 2 ISOPENTENYL PYROPHOSPHATE ISOMERASES ANDENHANCED CAROTENOID ACCUMULATION IN A UNICELLULAR CHLOROPHYTE, Proceedings of the National Academy of Sciences of the United Statesof America, 95(19), 1998, pp. 11482-11488
The enzyme isopentenyl pyrophosphate (IPP) isomerase catalyzes the rev
ersible isomerization of IPP to produce dimethylallyl pyrophosphate, t
he initial substrate leading to the biosynthesis of carotenoids and ma
ny other long-chain isoprenoids. Expression of IPP isomerase, and of t
wo enzymes specific to the carotenoid pathway (lycopene beta-cyclase a
nd beta-carotene-C-4-oxygenase), was followed in the green unicellular
alga Haematococcus pluvialis after exposure to high illumination, Thi
s alga uniquely accumulates carotenoids in the cytoplasm and in late d
evelopmental stages turns deep-red in color because of accumulation of
ketocarotenoids in the cytosol, The carotenoid/chlorophyll ratio incr
eased 3-fold in wild type and 6-fold in a precocious carotenoid-accumu
lating mutant (Car-3) within 24 h after increasing the illumination fr
om 20 to 150 mu mol photon m(-2).s(-1). Two cDNAs encoding PPP isomera
se in Haematococcus, ipiHp1 and ipiHp2, were identified. Although othe
rwise highly similar (95% identity overall), the predicted sequence of
ipiHp1 contained a 12-aa region not found in that of ipiHp2, This was
reflected by a size difference between two polypeptides of 34 and 32.
5 kDa, both of which reacted with an antibody to the product of ipiHp1
. We suggest that the 32.5-kDa form is involved with the carotenoid ac
cumulation in the cytoplasm, since the 32.5-kDa polypeptide was prefer
entially up-regulated by high light preceding the carotenoid increase
and only this form was detected in red cysts.