Cyanobacterial photorespiration as possible evolutionary origin for the plant C2 cycle

点击翻译按钮获取中文摘要

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.