Cyanobacterial photorespiration as possible evolutionary origin for the plant C2 cycle

光呼吸2-磷酸乙醇酸（2PG）代谢被发现是蓝藻光合作用所必需的，正如之前对植物所显示的那样。与植物相比，模式蓝藻胞囊藻6803中存在三种2PG代谢途径。除了植物的光呼吸C2循环外，它们还具有细菌甘油途径，能够通过草酸盐完全脱羧乙醛酸盐。完全阻断2PG代谢的突变体表现出高二氧化碳需求（HCR）表型。系统发育分析表明，植物光呼吸C2循环是蓝藻和变形菌代谢酶的混合物。对所选酶（如乙醇酸氧化酶）的生化分析表明，这种光呼吸酶也起源于蓝藻，但它在现存蓝藻中起着固定二氮的重要作用。在未来，我们将进行功能和进一步的系统发育分析，包括其他蓝藻菌株（如Nostoc sp. PCC 7120）和真核藻类。这些研究将阐明是否存在2PG代谢的替代途径以及它们的重要性。一个关键的问题是，甘油途径是否从蓝藻转移到藻类，如果它仍然在那里起作用。此外，2PG磷酸酶的获得对于从乙醇酸代谢进化到光呼吸至关重要。因此，将在蓝藻、藻类和植物中研究这些酶的系统发育、多样性和功能。

The photorespiratory 2-phosphoglycolate (2PG) metabolism was found to be essential for photosynthesis in cyanobacteria as was shown before for plants. In contrast to plants, three routes for 2PG metabolism are present in the model cyanobacterium Synechocystis 6803. In addition to the photorespiratory C2 cycle of plants, they also possess the bacterial glycerate pathway and are able to completely decarboxylate glyoxylate via oxalate. Mutants with a complete blockage of the 2PG metabolism exhibited a high-CO2-requiring (HCR) phenotype. Phylogenetic analyses revealed that the plant photorespiratory C2 cycle is a mixture of enzymes from cyanobacterial and proteobacterial metabolism. Biochemical analyses of selected enzymes, e.g. glycolate oxidases revealed that this photorespiratory enzyme also originated from cyanobacteria, but it performs an essential function in dinitrogen fixation in extant cyanobacteria. In the future we will perform functional and further phylogenetic analyses including other cyanobacterial strains (e.g. Nostoc sp. PCC 7120) and eukaryotic algae. These studies will clarify whether or not alternative routes for 2PG metabolism exist and how important they are. One key question is whether the glycerate pathway was transferred from cyanobacteria into algae and if it is still functioning there. Moreover, the acquisition of 2PG phosphatases was crucial for the evolution from glycolate metabolism into photorespiration. Accordingly, the phylogeny, diversity and function of these enzymes will be studied in cyanobacteria, algae, and plants.