DEDICATED ROLES OF PLASTID TRANSKETOLASES DURING THE EARLY-ONSET OF ISOPRENOID BIOGENESIS IN PEPPER FRUITS

Isopentenyl diphosphate (IPP), which is produced from mevalonic acid o r other nonmevalonic substrates, is the universal precursor of isopren oids in nature. Despite the presence of several isoprenoid compounds i n plastids, enzymes of the mevalonate pathway leading to IPP formation have never been isolated or identified to our knowledge. We now descr ibe the characterization of two pepper (Capsicum annuum L.) cDNAs, Cap TKT1 and CapTKT2, that encode transketolases having distinct and dedic ated specificities. CapTKT1 is primarily involved in plastidial pentos e phosphate and glycolytic cycle integration, whereas CapTKT2 initiate s the synthesis of isoprenoids in plastids via the nonmevalonic acid p athway. From pyruvate and glyceraldehyde-3-phosphate, CapTKT2 catalyze s the formation of 1-deoxy-xylulose-5-phosphate, the IPP precursor. Ca pTKT1 is almost constitutively expressed during the chloroplast-to-chr omoplast transition, whereas CapTKT2 is overexpressed during this peri od, probably to furnish the IPP necessary for increased carotenoid bio synthesis. Because deoxy-xylulose phosphate is shared by the plastid p athways of isoprenoid, thiamine (vitamin B-1), and pyridoxine (vitamin B-6) biosynthesis, our results may explain why albino phenotypes usua lly occur in thiamine-deficient plants.