Abstract
Apicomplexan parasites are known to possess highly reduced mitochondrial genomes encoding only three protein genes (cox1, cox3, cob) and fragmented rRNA genes. We investigated the mitochondrion of alveolate algae Chromera velia and Vitrella brassicaformis, the closest known phototrophic relatives to Apicomplexa. Genome, transcriptome and enriched mtDNA fractions were sequenced using Illumina platform. It appears that while Vitrella displays the same three mitochondrial protein genes as apicomplexan mitochondrion, Chromera mitochondrion holds a genome with the smallest known coding capacity containing just two protein genes (cox1, cox3) and fragmented rRNA genes. Consequently, the respiratory chain is reduced by the absence of complexes I (NADH:ubiquinone oxidoreductase) and III (ubiquinol: cytochrome c oxidoreductase). The respiratory chain is broken into two parts functioning independently. Electrons from the first part (complex II, alternative NADH dehydrogenases, dihydroorotase, electron-transfer flavoprotein etc.) are channeled to ubiquinone, which passes them to alternative oxidase, an electron sink, without any consequent proton pumping and link to the respiration whatsoever. Electrons for complex IV are provided by unique combination of L and D lactate cytochrome c oxidoreductases. In contrast, Vitrella still has the complex III but alternative enzymes are already available in the genome. It appears that most of non-canonical enzymes involved in the respiratory chain, which were found in chromerid algae, are of early eukaryotic origins.