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Euglenids are a group of algae of great interest for biotechnology, with a large and complex metabolic capability. To study the metabolic network, it is necessary to know the subcellular locations of the component enzymes, but despite a long history of research into Euglena, the subcellular locations of many major pathways are only poorly defined. Euglena is phylogenetically distant from other commonly studied algae, they have secondary plastids bounded by three membranes, and they can survive after destruction of their plastids. These unusual features make it difficult to assume that the subcellular organization of the metabolic network will be equivalent to that of other photosynthetic organisms. Moreover, we show here that the presence of the secondary chloroplast means that it is not possible to make reliable predictions of the subcellular locations of enzymes in Euglena using existing informatics tools. In order to generate a model of the central metabolic pathway operating in Euglena we analysed biochemical and proteomic information from a variety of sources to assess the subcellular location of relevant enzymes. We use these assignments to propose the compartmentation of the core metabolic pathways in Euglena, a prerequisite for the further study of the metabolic network of Euglena. This model of the metabolic network shows that, other than photosynthesis, all major pathways present in the chloroplast are duplicated elsewhere in the cell, and that several biosynthetic pathways confined to plastids in higher plants are localized elsewhere in Euglena. Our model demonstrates how this organism can synthesise all the metabolites required for growth from simple carbon inputs, and can survive in the absence of chloroplasts.

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