Graduate School of Science and Technology, Shizuoka University, Suruga-ku, Shizuoka, 422-8529 Japan.
Department of Biological Science, Faculty of Science, Shizuoka University, Shizuoka, 422-8529 Japan.
Plant Cell Physiol. 2020 Sep 1;61(9):1661-1668. doi: 10.1093/pcp/pcaa090.
Monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are the major components of thylakoid membranes and well-conserved from cyanobacteria to chloroplasts. However, cyanobacteria and chloroplasts synthesize these galactolipids using different pathways and enzymes, but they are believed to share a common ancestor. This fact implies that there was a replacement of the cyanobacterial galactolipid biosynthesis pathway during the evolution of a chloroplast. In this study, we first replaced the cyanobacterial MGDG biosynthesis pathway in a model cyanobacterium, Synechococcus elongatus PCC 7942, with the corresponding plant-type pathway. No obvious phenotype was observed under the optimum growth condition, and the content of membrane lipids was not largely altered in the transformants. We next replaced the cyanobacterial DGDG biosynthesis pathway with the corresponding plant-type pathway using the strain described above and isolated the strain harboring the replaced plant-type pathway instead of the whole galactolipid biosynthesis pathway. This transformant, SeGPT, can grow photoautotrophically, indicating that cyanobacterial galactolipid biosynthesis pathways can be functionally complemented by the corresponding plant-type pathways and that the lipid products MGDG and DGDG, and not biosynthesis pathways, are important. While SeGPT does not show strong growth retardation, the strain has low cellular chlorophyll content but it retained a similar oxygen evolution rate per chlorophyll content compared with the wild type. An increase in total membrane lipid content was observed in SeGPT, which was caused by a significant increase in DGDG content. SeGPT accumulated carotenoids from the xanthophyll groups. These results suggest that cyanobacteria have the capacity to accept other pathways to synthesize essential components of thylakoid membranes.
单半乳糖二酰基甘油 (MGDG) 和双半乳糖二酰基甘油 (DGDG) 是类囊体膜的主要成分，从蓝藻到叶绿体都得到很好的保守。然而，蓝藻和叶绿体使用不同的途径和酶合成这些半乳糖脂，但它们被认为有共同的祖先。这一事实表明，在叶绿体的进化过程中，蓝藻的半乳糖脂生物合成途径发生了取代。在这项研究中，我们首先在模式蓝藻 Synechococcus elongatus PCC 7942 中用相应的植物型途径取代了蓝藻的 MGDG 生物合成途径。在最佳生长条件下，没有观察到明显的表型，转化体中膜脂的含量也没有大幅改变。接下来，我们使用上述菌株用相应的植物型途径取代了蓝藻的 DGDG 生物合成途径，并分离出了取代了整个半乳糖脂生物合成途径的菌株。该转化体 SeGPT 可以进行光自养生长，表明蓝藻半乳糖脂生物合成途径可以被相应的植物型途径功能互补，并且脂类产物 MGDG 和 DGDG 而不是生物合成途径是重要的。虽然 SeGPT 没有表现出强烈的生长迟缓，但该菌株的细胞叶绿素含量较低，但与野生型相比，每单位叶绿素含量的氧气释放率保持相似。在 SeGPT 中观察到总膜脂含量增加，这是由于 DGDG 含量的显著增加所致。SeGPT 从叶黄素组中积累类胡萝卜素。这些结果表明，蓝藻有能力接受其他途径来合成类囊体膜的必需成分。
