Mitosome is a name given to the organelle of mitochondrial origin present in type I "amitochondrial" eukaryotes. It has been described in Entamoeba histolytica and later in microsporidian Trachipleistophora hominis. Presence of mitosome as well as genes of mitochondrial origin in genomes of these organisms suggests that contemporary amitochondriates ones harboured mitochondria, and do not represent lineages, which evolved before acquisition of mitochondria. Recently, mitosome was identified also in Giardia intestinalis, an amitochondrial protist, which was considered to be early branching primary amitochondrial eukaryote. The mitochondrial origin of mitosome is supported by (i) presence of two membranes surrounding these organelles, (ii) localization of proteins of iron sulfur cluster assembly machinery within these organelles (IscU, IscS, ferredoxin), (iii) targeting of proteins into the mitosome by means of N-terminal leader sequences with similar properties of mitochondrial leader sequences, (iv) FeS cluster assembly activity idetified in mitosome-rich cell fraction.
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