Dual-localized enzymatic components that constitute the mitochondrial and plastidial fatty acid synthase systems

ABSTRACT We report the identification and characterization of genes encoding three enzymes that are shared between the mitochondrial and plastidial-localized Type II fatty acid synthase systems (mtFAS and ptFAS, respectively). Two of these enzymes, β-ketoacyl-ACP reductase (pt/mtKR) and enoyl-ACP reductase (pt/mtER) catalyze two of the reactions that constitute the core, 4-reaction cycle of the FAS system, which iteratively elongate the acyl-chain by 2-carbon atoms per cycle. The third enzyme, malonyl-CoA:ACP transacylase (pt/mtMCAT) catalyzes the reaction that loads the mtFAS system with substrate, by malonylating the phosphopantetheinyl cofactor of acyl carrier protein (ACP). GFP-transgenic experiments determined the dual localization of these enzymes, which were validated by the characterization of mutant alleles, which were transgenically rescued by transgenes that were singularly retargeted to either plastids or mitochondria. The singular retargeting of these proteins to plastids rescued the embryo-lethality associated with disruption of the essential ptFAS system, but these rescued plants display phenotypes typical of the lack of mtFAS function. Specifically, these phenotypes include reduced lipoylation of the H subunit of the glycine decarboxylase complex, the hyperaccumulation of glycine, and reduced growth; all these traits are reversible by growing these plants in an elevated CO 2 atmosphere, which suppresses mtFAS-associated, photorespiration-dependent chemotypes.