Eukaryotic Cell doi:10.1128/EC.00393-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.
Genomic and biochemical analysis of lipid biosynthesis in the unicellular rhodophyte Cyanidioschyzon merolae: Lack of plastidic desaturation pathway results in the ‘coupled pathway’ of galactolipid synthesis
Naoki Sato*
and
Takashi Moriyama
Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan
* To whom correspondence should be addressed. Email:
naokisat{at}bio.c.u-tokyo.ac.jp.
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Abstract |
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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 prokaryotic and eukaryotic pathways, which are located within the plastids and the endoplasmic reticulum, respectively. Here we report results of biochemical as well as genomic analysis of lipids and fatty acids in the unicellular rhodophyte Cyanidioschyzon merolae. All the glycerolipids usually found in photosynthetic algae were found, such as mono- and digalactosyl diacylglycerol, sulfolipid, phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol. However, fatty acid composition was extremely simple. Only palmitic, stearic, oleic and linoleic acids were found as major acids. In addition, 3-trans-hexadecenoic acid was found as a very minor component in phosphatidylglycerol. Unlike 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 the 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 including plants and green algae.
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