Catalyzes cis-prenyl chain elongation to produce the polyprenyl backbone of dolichol, a glycosyl carrier-lipid required for the biosynthesis of several classes of glycoprotein. (2E,6E)-farnesyl diphosphate + n isopentenyl diphosphate = di-trans,poly-cis-polyprenyl diphosphate + n diphosphate Expressed in leaf trichomes and stem trichomes (PubMed:23134568). Expressed at low levels in young leaves, stems and old leaves (PubMed:23134568). Belongs to the UPP synthase family.