Metal ion cofactors are necessary for prenyltransferase enzymes. Magnesium and manganese can be used as metal ion cofactor by rubber transferase (a cis-prenyltransferase) associated with purified rubber particles. The rubber initiation rate, biosynthetic rate, and molecular weight produced in vitro from Hevea brasiliensis rubber transferase is regulated by metal ion concentration. In addition, K-m(IPP.Mg) varies significantly with [Mg2+].K-m(IPP.Mg) decreases from 8000 +/- 600 muM at [Mg2+] = 4 mM to 68 +/- 10 muM at [Mg2+] = 8 mM and increases back to 970 +/- 70 muM at [Mg2+] = 30 mM. The highest affinity of rubber transferase for IPP.Mg occurred when [Mg2+] = A(max) (metal concentration that gives highest IPP incorporation rate). A metal ion is required for rubber biosynthesis, but an excess of metal ions interacts with the rubber transferase inhibiting its activity. The results suggest that H. brasiliensis could use [Mg2+] as a regulatory mechanism for rubber biosynthesis and molecular weight in vivo.