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In eukaryotes (fungi, mammals, and plants), mitochondrial F1Fo-ATP synthase is composed of at least 16 highly-conserved subunits. Biochemical and computational analyses revealed that the enzyme from 3 Chlorophycean algae (C. reinhardtii, Polytomella sp. and Volvox carteri, belonging to the Chlamydomonadales order) lacks 8 subunits conserved in other eukaryotes which participate in building the peripheral stalk and in the dimerization of the enzyme. Instead, the algal enzyme contains 9 subunits of unknown evolutionary origin which were named ASA1 to 9 for "ATP Synthase Associated" proteins and which are probably involved in the stator architecture and in stabilizing a dimeric ATP synthase. The exact role of the ASA subunits in the enzyme activity and assembly is still poorly understood. In this work, we investigated the role of ASA7, a 19.5-kDa hydrophilic protein. First, we tried to determine whether this subunit was present in other green algal lineages. While a classical composition can be deduced for Ostreococcus tauri (Prasinophyceae) from its nuclear genome sequence, we found a sequence homolog to CrASA7 in Scenedesmus obliquus, a Sphaeropleale (Chlorophyceae). It remains to be demonstrated if S. obliquus ASA7 subunit really belongs to the mitochondrial ATP synthase, nevertheless, the presence of ASA7 gene in two different orders of Chlorophyceae (Chlamydomonadales and Sphaeropleales) led us to hypothesize that this unusual subunit may be characteristic of this lineage. The function of ASA7 was then analysed in Chlamydomonas by inactivation of the expression of the corresponding gene by RNAi. We isolated one clone showing reduced amount of ASA7 transcript, in parallel to a diminished dimeric ATP synthase content. The impact of this mutation on bioenergetics will be discussed.
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