MOLECULAR-CLONING OF A CDNA-ENCODING 3-KETOACYL-ACYL CARRIER PROTEIN SYNTHASE-III FROM LEEK

3-Ketoacyl-acyl carrier protein synthase III (KAS III) catalyzes the i nitial condensation of malonyl-acyl carrier protein (ACP) with acetyl- CoA in plant and bacterial fatty acid biosynthesis. The first cDNA clo ne encoding KAS III from a monocot is reported here. A cDNA clone was isolated from a leek epidermal cDNA library by screening with spinach and Arabidopsis heterologous probes from KAS III cDNA clones. When exp ressed in Escherichia coli, the cloned enzyme was able to catalyze the expected condensation reaction, was insensitive to cerulenin (100 mu M) and cross-reacted with spinach KAS III antibody. The 1476-bp cDNA c lone contained a 1206-bp open reading frame which encoded a 402-amino acid polypeptide. The deduced amino acid sequence showed significant s imilarities to other KAS IIIs although the leek sequence had some nota ble differences in regions otherwise completely conserved in dicots. N orthern blot analyses indicated that KAS III transcript levels were si milar in leaf epidermis and parenchyma, although developmental changes in transcript levels differed between these two tissues. In addition, leek KAS III was expressed in a manner comparable to leek oleoyl-ACP thioesterase (OTE), another enzyme of fatty acid biosynthesis, in both tissues.