The sequence reactions initially involved in fatty acid biosynthesis in plants must be reevaluated in light of the discovery that there is in spinach a previously unreported 3- ketoacyl-ACP synthase (sc-KAS). This enzyme is specific for short chain fatty acids and can be distinguished from other 3- ketoacyl-ACP synthases by its sensitivity to the antibiotics cerulenin and thiolactomycin. Based on work carried out with E. coli, as well as this work with spinach, there is strong evidence that this enzyme replaces in this sequence the enzyme acetyl transacylase, a proposed rate-limiting reaction in fatty acid synthesis. The objectives of this proposal are an extensive biochemical analysis of sc-KAS from spinach and isolation of the structural gene for sc-KAS. While the regulation of fatty acid biosynthesis in plants is poorly understood, there is in vitro evidence that the initial reactions of the sequence can influence the fatty acid composition of the products. The sc-KAS may play a critical role in this process and be the key to our understanding of regulation. For the past 25 years, fatty acid biosynthesis has been illustrated as a series of reactions catalyzed by the enzyme complex, fatty acid synthase. According to the pathway, the enzyme, acetyl transacylase is a key rate-limiting enzyme in this reaction. However, the discovery of the enzyme, 3-ketoacyl-ACP synthase, is challenging this series of reactions. It is believed that the latter enzyme is responsible for the step currently thought to be catalyzed by acetyl transacylase, contrary to the accepted dogma of fatty acid biosynthesis. This is a significant discovery that will be investigated through purification,characterization, and examination of the regulation of this enzyme. Understanding the role of this enzyme in fatty acid synthesis is of importance, particularly in the production of oilseed crops. If expression of this enzyme can be controlled through genetic engineering, there is the potential for over- production of fatty acids. Even if only a small increase in oil content could be achieved, it would have a major economic impact.
