part of prenyltransferase activity carotene biosynthetic process 15-cis-phytoene synthase activity https://github.com/geneontology/go-ontology/issues/21165 https://github.com/geneontology/go-ontology/issues/29285 prephytoene-diphosphate synthase activity GO:0046905 geranylgeranyl-diphosphate:geranylgeranyl-diphosphate geranylgeranyltransferase (15-cis-phytoene-forming) activity Catalysis of the reaction: 2 geranylgeranyl diphosphate = 15-cis-phytoene + 2 diphosphate. RHEA:34475 MetaCyc:RXN-13323 molecular_function geranylgeranyl-diphosphate geranylgeranyltransferase activity PSase activity EC:2.5.1.32 phytoene synthase activity