Proceedings of the NATIONAL ACADEMY OF SCIENCES

Introduction.-It has been shown earlierl 2 that the synthesis of the isoprenoid, rubber, by a rubber-bearing plant, guayule (Parthenium argentatum), involves acetate as an, intermediate. Not only does the administration of acetate to this plant result in a net increase in the amount of rubber formed, but, in addition, rubber synthesized by plants supplied with doubly C"4-labeled acetate possesses the same specific activity as the acetate given, indicating that all the carbon atoms of the rubber derive from and channel through acetate.3 Rubber, like other isoprenoids, is made up of repeating units of isoprene, which has the five-carbon branched-chain structure shown below. This structure appears, then, to be derived in the plant from acetate. It has also been shown in earlier workl 2 that the five-carbon acid d-methyl crotonate (BMC) is able to replace acetate in supporting rubber formation and is, in fact, more effective than acetate in this function. BMC, which is known as a naturally occurring plant constituent,4 has been shown to become radioactive in plant tissues supplied with C14-labeled acetate,3 and it has been suggested that BMC arises from acetate. This paper is concerned with the pathway by which BMC is synthesized from acetate by a plant enzyme system. The data to be presented suggest that the reactions involved follow the general course outlined below. Route of Synthesis of fl-Methyl Crotonate in a Plant System CH3COOH + CoA =CH3CO-CoCA Acetate Acetyl-CoA