Catalyzes the reaction from prephytoene diphosphate to phytoene. Catalyzes the condensation of isopentenyl diphosphate (IPP) with allylic pyrophosphates generating different type of terpenoids. 2 (2E,6E,10E)-geranylgeranyl diphosphate = 15-cis-phytoene + 2 diphosphate Binds 2 magnesium ions per subunit. Carotenoid biosynthesis; phytoene biosynthesis; all-trans-phytoene from geranylgeranyl diphosphate: step 1/1. Homodimer. In the N-terminal section; belongs to the phytoene/squalene synthase family. In the C-terminal section; belongs to the UPP synthase family.