In this study, we describe the mitochondrial genome of the excavate flagellate Euglena gracilis. Its gene complement is reduced as compared with the well-studied sister groups Diplonemea and Kinetoplastea. We have identified seven protein-coding genes: Three subunits of respiratory complex I (nad1, nad4, and nad5), one subunit of complex III (cob), and three subunits of complex IV (cox1, cox2, and a highly divergent cox3). Moreover, fragments of ribosomal RNA genes have also been identified. Genes encoding subunits of complex V, ribosomal proteins and tRNAs were missing, and are likely located in the nuclear genome. Although mitochondrial genomes of diplonemids and kinetoplastids possess the most complex RNA processing machineries known, including trans-splicing and editing of the uridine insertion/deletion type, respectively, our transcriptomic data suggest their total absence in E. gracilis. This finding supports a scenario in which the complex mitochondrial processing machineries of both sister groups evolved relatively late in evolution from a streamlined genome and transcriptome of their common predecessor.

Institute of Parasitology Biology Centre Czech Academy of Sciences České Budějovice Czech Republic

Institute of Parasitology Biology Centre Czech Academy of Sciences České Budějovice Czech Republic Canadian Institute for Advanced Research Toronto Ontario Canada

Institute of Parasitology Biology Centre Czech Academy of Sciences České Budějovice Czech Republic Departments of Biochemistry and Genetics Faculty of Natural Sciences Comenius University Bratislava Slovakia

Institute of Parasitology Biology Centre Czech Academy of Sciences České Budějovice Czech Republic Life Science Research Centre Faculty of Science University of Ostrava Ostrava Czech Republic

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