Catalyzes the condensation of isopentenyl pyrophosphate (IPP) with (2E,6E)-farnesyl diphosphate (E,E-FPP) to yield geranylgeranyl diphosphate (GGPP). (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. kcat is 1.0 min(-1). Isoprenoid biosynthesis; geranylgeranyl diphosphate biosynthesis; geranylgeranyl diphosphate from farnesyl diphosphate and isopentenyl diphosphate: step 1/1. Contains two aspartate-rich DDxxD motifs, designated as FARM (the first aspartate-rich motif) and SARM (the second aspartate-rich motif). Belongs to the FPP/GGPP synthase family.