Proposed to be involved in rubber biosynthesis as a particle-bound rubber transferase responsible for the cis-1,4-polymerization of isoprene subunits. Probably requires additional factors for the production of high molecular mass rubber. (cis-prenyl)n-diphosphate + isopentenyl diphosphate = (cis-prenyl)(n+1)-diphosphate + diphosphate Predominnatly expressed in latex. Belongs to the UPP synthase family.