In plant and microorganisms, aspartate semialdehyde dehydrogenase (ASDH) produces the branch point intermediate between the lysine and threonine/methionine pathways. In this study, we report the first cDNA cloning, purification, and characterization of a plant ASDH. The Arabidopsis thaliana ASDH is an homodimeric enzyme composed of subunits of 36 kDa. The plant enzyme exhibited a specific activity of 26 mumol NADPH oxidized min(-1) mg(-1) of protein with a K-M value for NADPH of 92 muM. ASDH showed cooperative behavior for aspartyl phosphate with a K-0.5 value of 37 muM. (C) 2002 Elsevier Science (USA).