Cyanobacterial photorespiration as possible evolutionary origin for the plant C2 cycle

光呼吸2-磷酸乙醇酸（2 PG）代谢被发现是必不可少的光合作用在蓝藻之前所示的植物。与植物相反，在模式蓝藻集胞藻6803中存在三种2 PG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酸途径，并且能够通过草酸完全脱羧乙醛酸。2 PG代谢完全阻断的突变体表现出高CO2需求（HCR）表型。系统发育分析表明，植物光呼吸C2循环是蓝藻和蛋白细菌代谢酶的混合物。生物化学分析选定的酶，如乙醇酸氧化酶显示，这种光呼吸酶也起源于蓝藻，但它在现存的蓝藻固氮中发挥着重要作用。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（如念珠藻属PCC 7120）和真核藻类。这些研究将阐明是否存在2 PG代谢的替代途径以及它们的重要性。一个关键问题是甘油酸途径是否从蓝藻转移到藻类，以及它是否仍然在那里发挥作用。此外，2 PG磷酸酶的收购是至关重要的进化从乙醇酸代谢到光呼吸。因此，将在蓝藻、藻类和植物中研究这些酶的发生、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.