Sato Naoki, Moriyama Takashi
Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan.
Eukaryot Cell. 2007 Jun;6(6):1006-17. doi: 10.1128/EC.00393-06. Epub 2007 Apr 6.
The acyl lipids making up the plastid membranes in plants and algae are highly enriched in polyunsaturated fatty acids and are synthesized by two distinct pathways, known as the prokaryotic and eukaryotic pathways, which are located within the plastids and the endoplasmic reticulum, respectively. Here we report the results of biochemical as well as genomic analyses of lipids and fatty acids in the unicellular rhodophyte Cyanidioschyzon merolae. All of the glycerolipids usually found in photosynthetic algae were found, such as mono- and digalactosyl diacylglycerol, sulfolipid, phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol. However, the fatty acid composition was extremely simple. Only palmitic, stearic, oleic, and linoleic acids were found as major acids. In addition, 3-trans-hexadecanoic acid was found as a very minor component in phosphatidylglycerol. Unlike the case for most other photosynthetic eukaryotes, polyenoic fatty acids having three or more double bonds were not detected. These results suggest that polyunsaturated fatty acids are not necessary for photosynthesis in eukaryotes. Genomic analysis suggested that C. merolae lacks acyl lipid desaturases of cyanobacterial origin as well as stearoyl acyl carrier protein desaturase, both of which are major desaturases in plants and green algae. The results of labeling experiments with radioactive acetate showed that the desaturation leading to linoleic acid synthesis occurs on phosphatidylcholine located outside the plastids. Monogalactosyl diacylglycerol is therefore synthesized by the coupled pathway, using plastid-derived palmitic acid and endoplasmic reticulum-derived linoleic acid. These results highlight essential differences in lipid biosynthetic pathways between the red algae and the green lineage, which includes plants and green algae.
构成植物和藻类质体膜的酰基脂质高度富含多不饱和脂肪酸，由两条不同的途径合成，即原核途径和真核途径，分别位于质体和内质网中。在此，我们报告了对单细胞红藻梅氏嗜热栖热菌中脂质和脂肪酸的生化及基因组分析结果。我们发现了光合藻类中常见的所有甘油脂质，如单半乳糖基二酰基甘油、硫脂、磷脂酰甘油、磷脂酰胆碱、磷脂酰乙醇胺和磷脂酰肌醇。然而，其脂肪酸组成极其简单。仅发现棕榈酸、硬脂酸、油酸和亚油酸为主要脂肪酸。此外，在磷脂酰甘油中发现3-反式十六烷酸为极微量成分。与大多数其他光合真核生物不同，未检测到具有三个或更多双键的多烯脂肪酸。这些结果表明，多不饱和脂肪酸对于真核生物的光合作用并非必需。基因组分析表明，梅氏嗜热栖热菌缺乏蓝细菌来源的酰基脂质去饱和酶以及硬脂酰酰基载体蛋白去饱和酶，而这两种酶在植物和绿藻中都是主要的去饱和酶。用放射性乙酸进行的标记实验结果表明，导致亚油酸合成的去饱和作用发生在质体外的磷脂酰胆碱上。因此，单半乳糖基二酰基甘油是通过偶联途径合成的，利用质体衍生的棕榈酸和内质网衍生的亚油酸。这些结果突出了红藻与包括植物和绿藻在内的绿色谱系在脂质生物合成途径上的本质差异。
