A MUTANT OF NANNOCHLOROPSIS DEFICIENT IN EICOSAPENTAENOIC ACID PRODUCTION

The most abundant fatty acids found in Nannochloropsis are 16:0, 16:1 (n-7) and 20:5 (n-3). By analysing the fatty acid composition of mutag enized cells by gas chromatography, we discovered a mutant strain of N annochloropsis that is completely devoid of 5,8,11,14,17-cis-eicosapen taenoic acid [20:5 (n-3)]. An analysis of total fatty acids from whole cells showed that the loss of 20:5 (n-3) in mutant cells was accompan ied by a concomitant increase in the relative amount of 20:4 (n-6). Ho wever, the increase in 20:4 (n-6) was not proportional to the loss of 20:5 (n-3) within individual lipid classes. The percentage of 20:4 (n- 6) in monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol and phosphatidylglycerol (PG) in the mutant strain was lower than the wild type levels of 20:5 (n-3) in these lipids. Conversely, the percen tage of 20:4 (n-6) in phosphatidylcholine, phosphatidylethanolamine an d phosphatidylinositol in the mutant cells was higher than the wild ty pe levels of 20:5 (n-3). Molecular species analysis of MGDG showed tha t, compared to the wild type strain, the mutant strain contained more 16:1/16:0 MGDG, and less of the MGDG species containing C-20 fatty aci ds. Relatively low levels of 20:4/20:4 MGDG were found in the mutant, although 20:5/20:5 MGDG was abundant in wild type cells. Relative memb rane lipid levels were altered in the mutant compared to the wild type strain, particularly PG, which showed a fourfold decrease. The mutant cells grow more slowly than the wild type cells on both solid and in liquid media. Electron micrographs showed that the mutant strain had f ewer thylakoid stacks per chloroplast, and fewer thylakoids per stack than wild type cells. We concluded that the mutation affects an extrac hloroplastic (n-3) desaturase that is required for the synthesis of 20 :5 (n-3), which is found in both chloroplast and extrachloroplast lipi ds.