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The yeast ORF YPL060w/LPE10 encodes a homologue of the mitochondrial protein Mrs2p. These two proteins are 32% identical, and have two transmembrane domains in their C-terminal regions and a putative magnesium transporter signature, Y/F-G-M-N, at the end of one of these domains. Data presented here indicate that Lpe10p is inserted into the inner mitochondrial membrane with both termini oriented towards the matrix space. Disruption of the LPE10 gene results in a growth defect on non-fermentable substrates (petite phenotype) and a marked defect in group II intron splicing. The fact that in intron-less strains lpe10 disruptants also exhibit a petite phenotype indicates that functions other than RNA splicing are affected by the absence of Lpe10p. In the mitochondria, concentrations of magnesium, but not of several other divalent metal ions, are increased when Lpe10p is overexpressed and …