Generates tritrans,heptacis-undecaprenyl diphosphate from isopentenyl pyrophosphate (IPP) and geranylgeranyl diphosphate. It is probably the precursor of glycosyl carrier lipids. geranylgeranyl diphosphate + 7 isopentenyl diphosphate = 7 diphosphate + tri-trans,hepta-cis-undecaprenyl diphosphate Binds 2 magnesium ions per subunit. Homodimer. Belongs to the UPP synthase family.