Submitted to: Phytochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2006
Publication Date: 6/15/2006
Citation: Dacosta, B.M., Keasling, J.D., Mcmahan, C.M., Cornish, K. 2006. Magnesium ion regulation of the in vitro rubber biosynthesis by parthenium argentatum gray. Phytochemistry.
Interpretive Summary: Although there are over 2500 plant species that produce natural rubber, there are only a few species that produce the high molecular weight rubber necessary for high product quality and performance. Currently, the main commercial source of natural rubber is Hevea brasiliensis Muell. Arg. (Brazilian or Para rubber tree), which produces high molecular weight rubber obtained by tapping the laticifers in the tree bark. However, H. brasiliensis commercial production is limited to tropical regions of the world. Also, it is one of the most genetically uniform crops under cultivation, as it has a narrow breeding ancestry, and depends almost entirely on plantation-grown clonal trees (clonal scions grafted onto seeding root stocks), which makes it prone to pathogenic attack. Parthenium argentatum Gray (guayule), a native of the Chihuahuan desert of Mexico and Texas, is another plant species that produces high molecular weight rubber (Ray, 1993), and is an alternative source of natural rubber for commercial use P. argentatum is being commercially developed as a source of latex for medical products becuase its rubber particle-associated proteins do not cross react with lgE (Type l latex allergy) and lgG antibodies to H. brasiliensis latex proteins Here, we characterize the role of Mg2+ concentration in rubber biosynthesis by enzymatically active rubber particles purified from P. argentatum. The effect of [Mg2+] on initiation, biosynthetic rate, molecular weight and substrate affinity was determined and then compared to rubber biosynthesis in H. brasiliensis.
Technical Abstract: Natural rubber is produced by a rubber transferase (a cis-preny transferase). Rubber transferase uses allylic pyrophosphate to initiate the rubber molecule and isopentenyl pyrophosphate (IPP) to form the polymer. Rubber biosynthesis also requires a divalent metal cation. Understanding how molecular weight is regulated is important because high molecular weight is required for high quality rubber. We characterized the in vitro effects of Mg2+ on the biosynthetic rate of rubber produced by an alternative natural rubber crop, Parthenium argentatum (guayule). The affinity of the rubber transferase from P. argentatum for IPP'Mg was shown to depend on the Mg2+ concentration in a similar fashion to the H. brasiliensis rubber transferase, although to a less extreme degree. Also, in vitro Mg2+ concentration significantly affects rubber molecular weight of both species, but molecular weight is less sensitive to Mg2+ concentration in P. argentatum than in H. brasiliensis.