STRUCTURE, CHROMOSOMAL LOCATION AND EXPRESSION OF A RICE GENE ENCODING THE MICROSOME OMEGA-3-FATTY-ACID DESATURASE

The omega-3 fatty acid desaturases are membrane-bound enzymes catalyzi ng the conversion of linoleic acid to linolenic acid in lipids, and ar e located both in the microsome and plastid envelopes as two different isoforms. A cDNA encoding the microsome omega-3 fatty acid desaturase (OsFAD3) and the corresponding genomic clone were isolated from rice (Oryza sativa L.). The OsFAD3 gene was composed of 8 exons and 7 intro ns. A microsatellite was present in the second exon of the OsFAD3 gene , showing polymorphism between Indica and Japonica rice varieties. The mapping of this microsatellite showed that the OsFAD3 gene was locate d on chromosome II. Expression of the OsFAD3 cDNA in tobacco hairy roo t tissues and subsequent analysis of fatty acid compositions demonstra ted the activity of the microsome omega-3 fatty acid desaturase. The O sFAD3 mRNA was abundant in root tissues, but was hardly detectable in leaves. In root tissues, a high level of the OsFAD3 mRNA was observed at 15 degrees C and 20 degrees C, with its level decreasing markedly a t temperatures below 10 degrees C. The accumulation of the OsFAD3 mRNA in leaf tissues remained at quite low levels, both at normal growth t emperatures and at chilling temperatures. Similar temperature response s of the OsFAD3 gene were observed both in chilling- tolerant and in c hilling-intolerant rice cultivars.