3-KETOACYL-ACYL CARRIER PROTEIN SYNTHASE-III FROM SPINACH (SPINACIA-OLERACEA) IS NOT SIMILAR TO OTHER CONDENSING ENZYMES OF FATTY-ACID SYNTHASE

A cDNA clone encoding spinach (Spinacia oleracea) 3-ketoacyl-acyl carr ier protein synthase III (KAS III), which catalyzes the initial conden sing reaction in fatty acid biosynthesis, was isolated. Based on the a mino acid sequence of tryptic digests of purified spinach KAS III, deg enerate polymerase chain reaction (PCR) primers were designed and used to amplify a 612-bp fragment from first-strand cDNA of spinach leaf R NA. A root cDNA library was probed with the PCR fragment, and a 1920-b p clone was isolated. Its deduced amino acid sequence matched the sequ ences of the tryptic digests obtained from the purified KAS III. North ern analysis confirmed that it was expressed in both leaf and root. Th e clone contained a 1218-bp open reading frame coding for 405 amino ac ids. The identity of the clone was confirmed by expression in Escheric hia coli BL 21 as a glutathione S-transferase fusion protein. The dedu ced amino acid sequence was 48 and 45% identical with the putative KAS III of Porphyra umbilicalis and KAS III of E. coli, respectively. It also had a strong local homology to the plant chalcone synthases but h ad little homology with other KAS isoforms from plants, bacteria, or a nimals.