Practice question

crtI gene deployed in Golden Rice was sourced from soil bacterium Erwinia uredovora strain now taxonomically reclassified as Pantoea ananatis, a Gram-negative enterobacterium inhabiting plant surfaces capable of synthesizing carotenoids as photoprotective pigments. Gene encodes 489 amino acid phytoene desaturase with FAD binding domain catalyzing sequential introduction of four double bonds into phytoene backbone converting 15-cis-phytoene via phytofluene, zeta-carotene, neurosporene to all-trans lycopene with concomitant isomerization, effectively replacing plant PDS, ZDS, CRTISO functions. Bacterial enzyme simplified metabolic engineering reducing transgene count from three to two, avoiding homology-dependent gene silencing observed when introducing multiple plant desaturases sharing conserved domains. Codon usage adjusted for plant expression and fused to transit peptide for amyloplast import. In bacteria crtI operates without membrane association in cytosol, whereas in plants localized to plastid membranes where lipophilic substrates partition. Its broad substrate specificity and lack of feedback regulation increase flux. Isolation from bacteria exemplifies utilization of microbial modules to bypass plant regulatory constraints in biofortification.