MODIFICATION OF BRASSICA-NAPUS SEED OIL BY EXPRESSION OF THE ESCHERICHIA-COLI FABH GENE, ENCODING 3-KETOACYL-ACYL CARRIER PROTEIN SYNTHASE-III

The Escherichia coli fabH gene encoding 3-ketoacyl-acyl carrier protei n synthase III (KAS III) was isolated and the effect of overproduction of bacterial KAS III was compared in both E. coli and Brassica napus. The change in fatty acid profile of E. coli was essentially the same as that reported by Tsay et al. (J Biol Chem 267 (1992) 6807-6814), na mely higher C14:0 and lower C18:1 levels. In our study, however, an ar rest of cell growth was also observed. This and other evidence suggest s that in E. coli the accumulation of C14:0 may not be a direct effect of the KAS III overexpression, but a general metabolic consequence of the arrest of cell division. Bacterial KAS III was expressed in a see d- and developmentally specific manner in B. napus in either cytoplasm or plastid. Significant increases in KAS III activities were observed in both these transformation groups, up to 3.7 times the endogenous K AS III activity in mature seeds. Only the expression of the plastid-ta rgeted KAS III gene, however, affected the fatty acid profile of the s torage lipids, such that decreased amounts of C18:1 and increased amou nts of C18:2 and C18:3 were observed as compared to control plants. Su ch changes in fatty acid composition reflect changes in the regulation and control of fatty acid biosynthesis. We propose that fatty acid bi osynthesis is not controlled by one rate-limiting enzyme, such as acet yl-CoA carboxylase, but rather is shared by a number of component enzy mes of the fatty acid biosynthetic machinery.