Catalysis of the reaction: 2 geranylgeranyl diphosphate -> 15-cis-phytoene + 2 diphosphate.
Source:RHEA:34479,
GOC:ai,
PMID:12641468
Comment
Note that this should not be confused with 'geranylgeranyl-diphosphate geranylgeranyltransferase activity ; GO:0016767', EC:2.5.1.32. This activity is the second part of the formation of phytoene from geranylgeranyl-diphosphate, the first stage of which is catalyzed by EC:2.5.1.32.