Cloning of a Higher-Plant Plastid [omega]-6 Fatty Acid Desaturase cDNA and Its Expression in a Cyanobacterium

Abstract

Oligomers based on amino acids conserved between known plant [omega]-3 and cyanobacterium [omega]-6 fatty acid desaturases were used to screen an Arabidopsis cDNA library for related sequences. An identified clone encoding a novel desaturase-like polypeptide was used to isolate its homologs from Glycine max and Brassica napus. The plant deduced amino acid sequences showed less than 27% similarity to known plant [omega]-6 and [omega]-3 desaturases but more than 48% similarity to cyanobacterial [omega]-6 desaturase, and they contained putative plastid transit sequences. Thus, we deduce that the plant cDNAs encode the plastid [omega]-6 desaturase. The identity was supported by expression of the B. napus cDNA cyanobacterium. Synechococcus transformed with a chimeric gene that contains a prokaryotic promoter fused to the rapeseed cDNA encoding 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 cyanobacterium. The plastid and cytosolic homologs of plant [omega]-6 desaturases are much more distantly related than those of [omega]-3 desaturases.