Dehydrogenates carotenes in the cis conformation: has cis-to-trans isomerase activity and mediates dehydrogenation of cis-phytoene, producing zeta-carotene via the intermediary of phytofluene by the symmetrical introduction of 2 double bonds at the C-11 and C-11' positions of phytoene. 15-cis-phytoene + 2 A = 2 AH2 + all-trans-zeta-carotene Carotenoid biosynthesis; lycopene biosynthesis. Belongs to the carotenoid/retinoid oxidoreductase family.