Identification of neoxanthin synthase as a carotenoid cyclase paralog

Neoxanthin, a precursor of the plant hormone abscisic acid, is an allenic x anthophyll recognized as the last product of carotenoid synthesis in green plants. A cDNA for neoxanthin synthase (NSY) was isolated from tomato using a molecular approach based on the mechanistic and structural similarities of NSY to two other closely related carotenogenic enzymes, lycopene cyclase (LCY) and capsanthin-capsorubin synthase (CCS). The identified tomato NSY cDNA (T.NSY) encodes a 56-kDa plastid-targeted protein that when expressed in Escherichia coli, catalyzes the conversion of violaxanthin to neoxanthin . In tobacco leaves that transiently express T.NSY, an increase in neoxanth in content with a concomitant decrease in violaxanthin is observed. NSY is structurally similar to LCY and CCS. However, in Cyanobacteria, the general ly accepted progenitor of plastids, both CCS and NSY are absent while LCY i s present. LCY catalyzes a simplified version of the reaction catalyzed by NSY and CCS suggesting that these two enzymes were remodeled from LCY durin g higher plant evolution to create new forms of oxidized carotenoids.