Genes introduced in Golden rice include:
Development of Golden Rice elegantly reduced complex plant carotenoid desaturation steps to two transgenes. Phytoene synthase psy catalyzes committed head-to-head condensation of two geranylgeranyl diphosphate GGPP molecules to colorless 15-cis-phytoene, rate limiting step strongly regulated by light and feedback. Phytoene requires four desaturation steps and isomerization to become all-trans lycopene red pigment. Higher plants use phytoene desaturase, zeta-carotene desaturase, carotene isomerase ZDS, CRTISO separately. Bacterial crtI from Erwinia uredovora performs all reactions in single polypeptide with FAD cofactor, converting phytoene directly to lycopene via poly-cis intermediates. Combined psy plus crtI sufficient because endogenous rice endosperm possesses residual lycopene beta-cyclase and other downstream enzymes induced by product accumulation. Vectors used rice glutelin Gt1 promoter for endosperm specificity ensuring seed expression, CaMV 35S for crtI, and pea Rubisco small subunit transit peptide targeting to amyloplasts where isoprenoid precursor GGPP generated via MEP pathway. This two-gene strategy minimized gene silencing and transgene load while restoring entire pathway up to beta-carotene.
Ref: Beyer et al. J Nutr 2002 Golden Rice pathway; NCBI Metabolic pathway psy crtI mechanism.