Regulation of initiation and polymer molecular weight of cis-1,4-polyisoprene synthesized in vitro by particles isolated from Parthenium argentatum (Gray)

Parthenium argentatum is a prime candidate for development as a new crop for the production of natural rubber (cis-1,4-polyisoprene). The rubber obtained from P. argentatum is comparable in molecular weight (and qualify) to that obtained from Hevea brasiliensis (M-r ca. 10(6) Da), which is currently the sole commercial source of natural rubber. Most of the 2500 rubber-producing plant species known make much lower molecular weight rubber, although the regulation of polymer molecular weight is not understood. In the experiments reported here, we investigated the regulation of rubber biosynthesis and polymer molecular weight using purified, enzymatically-active rubber particles isolated from P. argentatum, under various concentrations of the allylic diphosphate (allylic-PP) rubber molecule initiators farnesyl diphosphate (FPP, a 15-carbon molecule), or geranyl diphosphate (GPP, a 10-carbon molecule) and the elongation substrate isopentenyl diphosphate (IPP). Our results show that the rates of both rubber molecule initiation and polymerization, and the final polymer molecular weight, were greatly affected by the concentration of initiator and IPP. Increasing allylic-PP initiator concentrations caused an increase in the amount of rubber synthesized and the number of molecules initiated but a decrease in the mean polymer molecular weight; increasing IPP concentrations increased the amount of rubber and the mean polymer molecular weight. At identical substrate concentrations of IPP and allylic-PP, GPP initiated about one third of the rubber polymers initiated by FPP but incorporated about two thirds the amount of IPP, compared to FPP. Consequently, while the amount of rubber synthesized in the presence of GPP was less than with FPP, the rubber polymers synthesized with GPP initiator were, on average, about twice the molecular weight of those with FPP. (C) 1999 Elsevier Science Ltd. All rights reserved.