DIFFERENTIAL EXPRESSION OF 2 ISOPENTENYL PYROPHOSPHATE ISOMERASES ANDENHANCED CAROTENOID ACCUMULATION IN A UNICELLULAR CHLOROPHYTE

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.