Catalyzes the sequential condensation of isopentenyl diphosphate (IPP) with geranylgeranyl diphosphate (GGPP) to yield (2Z,6Z,10Z,14Z,18Z,22Z,26Z,30E,34E,38E)-undecaprenyl diphosphate (tritrans,heptacis-UPP). 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. Extended N-terminus.