CLONING OF A HIGHER-PLANT PLASTID OMEGA-6 FATTY-ACID DESATURASE CDNA AND ITS EXPRESSION IN A CYANOBACTERIUM

Oligomers based on amino acids conserved between known plant omega-3 a nd cyanobacterium omega-6 fatty acid desaturases were used to screen a n Arabidopsis cDNA library for related sequences. An identified clone encoding a novel desaturase-like polypeptide was used to isolate its h omologs from Glycine max and Brassica napus. The plant deduced amino a cid sequences showed less than 27% similarity to known plant omega-6 a nd omega-3 desaturases but more than 48% similarity to cyanobacterial omega-6 desaturase, and they contained putative plastid transit sequen ces. Thus, we deduce that the plant cDNAs encode the plastid omega-6 d esaturase. The identity was supported by expression of the B. napus cD NA in cyanobacterium. Synechococcus transformed with a chimeric gene t hat contains a prokaryotic promoter fused to the rapeseed cDNA encodin g all but the first 73 amino acids partially converted its oleic acid fatty acid to linoleic acid, and the 16:1(9c) fatty acid was converted primarily to 16:2(9c,12) in vivo. Thus, the plant omega-6 desaturase, which utilizes 16:1(7c) in plants, can utilize 16:1(9c) in the cyanob acterium. The plastid and cytosolic homologs of plant omega-6 desatura ses are much more distantly related than those of omega-3 desaturases.