中文摘要：

球等鞭金藻（Isochrysis galbana）是一类单细胞海洋微藻，富含二十二碳六烯酸（DHA，22：6△^4,7,10,13,16,19）。我们利用RACE的方法从球等鞭金藻cDNA文库中同源克隆到一个大小为1329bp的cDNA片段，编码442个氨基酸的多肽，分子量约49．9kD。生物信息学分析表明，其编码产物N端具有细胞色素65结构域，以及与电子传递有关的三个富含组氨酸的结构域，与Pavlovasa linaA5去饱和酶同源性最高，达56％，故将该基因命名为IgD5。酿酒酵母功能鉴定实验表明，其编码的蛋白质具有A5去饱和酶活性，能够将二高-τ-亚麻酸fDGL20：3△^8，11,14）转化成花生四烯酸（AA，20：4△^5，8，11,14），转化效率平均为34．6％，最高可达40．3％。

英文摘要：

The very long chain （〉~C20） polyunsaturated fatty acids （VLCPUFA）, such as arachidonic acid （AA; 20：4036）, eicosapentaenoic acid （EPA; 20：5033） and docosapentaenoic acid （DHA; 22：6033） are essential for human health and nutrition. Dietary supplementation with these fatty acids is not only helpful for fetal neuronal, but also can reduce the risk of cardiovascular disease, hypertension, inflammatory and other diseases. Human beings can synthesize these fatty acids from the two essential fatty acid, linoleic acid （LA; 18：2036） and ct-linolenic acid （ALA; 18：3033）, which must be obtained from the diet. However, the synthetic efficiency is somewhat limited, and can not meet our daily requirement. AA can be obtained easily from meat, egg and milk, often in excessive amounts, EPA and DHA, however, can only be obtained from marine fish that are so often omitted from the modern diet, in much lower amounts than required. This shortage is further worsened by the fact that the natural marine fish resources have been depleting fast in recent years. In addition, the recent findings of toxic chemicals in fish oil has created fears for the consumption of fish products hence reduced the intake of EPA and DHA even further. Therefore, alternative source of these VLCPUFAs are therefore de- sirable. To obtain them from oil plants in commercial and sustainable quantities is particularly attractive. However, no oil-seed species produce such products naturally. The VLCPUFA biosynthesis pathways in organisms such as filamen- tous fungus and marine microalgae have been elaborated. Accordingly, genes encoding for elongases and desaturases involved in their metabolic pathway have been cloned from a variety of organisms including algae, mosses, fungi, nematodes and humans in the last 10 years. The reconstruction of the VLCPUFAs metabolic pathway into higher plants has been achieved in Arabidopsis, linseed, mustard and soya bean by introducing a set of 3--9 fatty acid desaturase and elongase genes. Howe