Two Arabidopsis thaliana carotene desaturases, phytoene desaturase and zeta-carotene desaturase, expressed in Escherichia coli, catalyze a poly-cis pathway to yield pro-lycopene.

We have expressed in Escerichia coli the enzymes geranylgeranyl diphosphate synthase and phytoene synthase, from the soil bacterium Erwinia stewartii, and the two carotene desaturases phytoene desaturase and carotene zeta-carotene desaturase from Arabidopsis thaliana. We show that pro-lycopene (7,9,7',9'-tetra-cis)-lycopene is the main end product of the plant desaturation pathway in these cells. In addition, light is required in this system. Whereas in the dark mainly zeta-carotene, the phytoene desaturase product, accumulates, illumination leads to activation of this intermediate caused by its photoisomerization. zeta-Carotene then meets the stereospecific requirements of zeta-carotene desaturase and pro-lycopene is formed. In contrast, a strain of E. coli carrying geranylgeranyl diphosphate synthase, phytoene desaturase and the bacterial carotene desaturase CrtI, which mediates lycopene formation from phytoene, does not require light, nor is a poly-cis-lycopene species formed. The stereoselectivity of the plant-type desaturation pathway expressed in E. coli is the same as previously shown with chromoplast membranes. As the phytoene desaturase and zeta-carotene desaturase used originate from a system not capable of developing chromoplasts, this indicates that the poly-cis pathway of carotene desaturation may have a wider occurrence than initially believed.

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