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 Protein modification; protein glycosylation. Expressed in low levels in the whole plant. Preferentially expressed in roots. Belongs to the UPP synthase family.