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.

中文翻译：

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用植物型路径延长蓝藻集球藻PCC 7942中的植物型途径完全替代半乳糖脂类生物合成途径。

单半乳糖基二酰基甘油（MGDG）和二半乳糖基二酰基甘油（DGDG）是类囊体膜的主要成分，并且从蓝细菌到叶绿体都非常保守。然而，蓝细菌和叶绿体使用不同的途径和酶来合成这些半乳糖脂，但据信它们具有相同的祖先。这一事实表明，在叶绿体的进化过程中，蓝藻半乳糖脂的生物合成途径被替换了。在这项研究中，我们首先取代了蓝藻MGDG生物合成途径中的模型蓝藻，细长聚球藻PCC 7942，具有相应的植物类型途径。在最佳生长条件下没有观察到明显的表型，并且在转化体中膜脂质的含量没有很大变化。接下来，我们使用上述菌株将蓝藻DGDG生物合成途径替换为相应的植物类型途径，并分离出具有替代植物类型途径而不是整个半乳糖脂生物合成途径的菌株。该转化体SeGPT可以自养生长，表明蓝细菌半乳糖脂类生物合成途径可以在功能上由相应的植物型途径进行补充，并且脂类产物MGDG和DGDG而不是生物合成途径很重要。尽管SeGPT没有显示出强烈的生长迟缓，该菌株的细胞叶绿素含量低，但与野生型相比，每叶绿素含量的氧释放速率相似。在SeGPT中观察到总膜脂质含量增加，这是由于DGDG含量显着增加所致。SeGPT积累了来自叶黄素组的类胡萝卜素。这些结果表明，蓝细菌具有接受其他途径合成类囊体膜基本成分的能力。