Lysine decarboxylase (LDC) catalyzes the first-step in the biosynthetic pathway of quinolizidine alkaloids (QAs), which form a distinct, large family of plant alkaloids. A cDNA of lysine/ornithine decarboxylase (L/ODC) was isolated by differential transcript screening in QA-producing and nonproducing cultivars of Lupinus angustifolius. We also obtained L/ODC cDNAs from four other QA-producing plants, Sophora flavescens, Echinosophora koreensis, Thermopsis chinensis, and Baptisia australis. These L/ODCs form a phylogenetically distinct subclade in the family of plant ornithine decarboxylases. Recombinant L/ODCs from QA-producing plants preferentially or equally catalyzed the decarboxylation of L-lysine and L-ornithine. L. angustifolius L/ODC (La-L/ODC) was found to be localized in chloroplasts, as suggested by the transient expression of a fusion protein of La-L/ODC fused to the N terminus of green fluorescent protein in Arabidopsis thaliana. Transgenic tobacco (Nicotiana tabacum) suspension cells and hairy roots produced enhanced levels of cadaverine-derived alkaloids, and transgenic Arabidopsis plants expressing (La-L/ODC) produced enhanced levels of cadaverine, indicating the involvement of this enzyme in lysine decarboxylation to form cadaverine. Site-directed mutagenesis and protein modeling studies revealed a structural basis for preferential LDC activity, suggesting an evolutionary implication of L/ODC in the QAproducing plants. INTRODUCTIONAlkaloids are one of the most diverse groups of natural products and are found in ;20% of plant species. Many of the ;12,000 known alkaloids produced by plants display potent pharmacological activities and several are widely used as pharmaceuticals (Croteau et al., 2000;De Luca and St Pierre, 2000). Alkaloids are derived from the products of primary metabolism with amino acids such as Phe, Tyr, Trp, Orn, and Lys serving as their main precursors (Facchini, 2001). Quinolizidine alkaloids (QAs) are derived from cadaverine, and several hundred structurally related compounds have been identified that are distributed mostly within the Leguminosae (Ohmiya et al., 1995;Michael, 2008). Some QAs exhibit beneficial pharmacological properties, such as cytotoxic, antiarrhythmic, oxytocic, hypoglycemic, and antipyretic activities, and thus can be used as drugs (Ohmiya et al., 1995). They can therefore serve as potential starting compounds in the development of new drugs and in pest control for plants .QAs are synthesized through the cyclization of the cadaverine unit (Golebiewski and Spenser, 1988), which is produced through the action of Lys decarboxylase (LDC; EC 4.1.1.18) (Figure 1), via a postulated enzyme-bound intermediate . The various skeletons of QAs [e.g., bicyclic alkaloids such as (2)-lupinine and (+)-epilupinine or tetracyclic alkaloids such as (+)-multiflorine, (+)-lupanine, and (+)-matrine] are then further modified by tailoring reactions (e.g., dehydrogenation, oxygenation, esterification, and glycosylation) to yield hundreds of structurally related alkaloids (Ohmiya ...