Involved in the biosynthesis of carotenoids. Catalyzes the condensation of two molecules of geranylgeranyl diphosphate (GGPP) to give prephytoene diphosphate (PPPP) and the subsequent rearrangement of the cyclopropylcarbinyl intermediate to yield 15-cis-phytoene. 2 (2E,6E,10E)-geranylgeranyl diphosphate = 15-cis-phytoene + 2 diphosphate ATP is required for the transferase activity but it does not seem to be hydrolyzed during the reaction. Carotenoid biosynthesis; phytoene biosynthesis. Belongs to the phytoene/squalene synthase family. The enzyme produces 15-cis-phytoene. The conversion to all-trans-phytoene is due to photoisomerisation.