MTO1 Mutations
(A) Electropherograms of MTO1 of Pt2 showing the c.1282G>A (left) and c.1858dup (right), both in heterozygosis.
(B) Real-time PCR on retrotranscribed cDNA from fibroblasts of individuals 1 (P1) and 2 (P2). The amount of MTO1 transcript (normalized to GAPDH levels) is comparable in mutant versus WT control samples (Ct), indicating no mRNA decay. Data are represented as mean ± SD.
(C) Western-blot analysis of MTO1. Top: schematic representation of the precursor WT MTO1 (isoform a), its mature species after cleavage of a predicted 25 aa mitochondrial targeting sequence in the N terminus, and the mature p.Arg620Lysfs^∗8 mutant species. Bottom: Western-blot analysis on isolated mitochondria. Retic.: reticulocyte lysate used for in-vitro protein synthesis; Prec.: in-vitro-translated 77 kDa MTO1 precursor protein (green arrowhead); Ct: isolated mitochondria from control fibroblasts; P2: isolated mitochondria from individual 2 fibroblasts; fs^∗8: in-vitro-translated 67 kDa mature protein carrying the truncating p.Arg620Lysfs^∗8 variant (red arrowhead); Mat.: in-vitro-translated 74 kDa WT mature MTO1 (black arrowhead). A faint crossreacting band is visualized in mt P2 sample (red asterisk), corresponding to the mature p.Arg620Lysfs^∗8 truncated protein. An unspecific signal is present in mt samples (gray asterisks). The position of the 72 kDa molecular weight marker protein is also indicated. SDHA was used as loading control.
(D) MRR measured in immortalized fibroblasts from Pt 2, in naive condition (P2) or overexpressing MTO1 (P2+MTO1), and in control subjects (Ct1, Ct2). MRR values are expressed as pMolesO₂/min/cells. Data are represented as mean ± SD. Two-tail, unpaired Student's t test was applied for statistical significance. ^∗∗∗: p < 0.001.

A-Site Structures of Yeast 15S and Human 12S rRNAs
Secondary structure of site A of the WT yeast strain 15S rRNA, of the P^R yeast strain 15S rRNA, of the WT human 12S rRNA, and of the mutant human 12S rRNA—the last two carrying either the 1555G or the 1494T mtDNA mutation, both associated with P^S. The pairing of 1477–1583 nucleotides in WT yeast rRNA corresponds to that of 1494-1555 nucleotides in mutant human rRNA and confers aminoglycoside susceptibility.

In-Vivo Mitochondrial Translation of P^Rmto1Δ Strain Transformed with WT or Mutant MTO1 Alleles
Mitochondrial gene products were labeled with [³⁵S] methionine in whole cells at 30°C in the presence of cycloheximide for 7 min. 19 Equivalent amounts of total cellular proteins were separated by SDS-PAGE on a 17.5% polyacrylamide gel, transferred to a nitrocellulose membrane, and exposed to X-ray film. The mto1Δ strain transformed with the empty vector (vector) contained 40% of petite cells, thus reducing the overall signal. Cox: cytochrome c oxidase; Cob: cytochrome b; Atp: ATP synthase; Var1: small mitochondrial ribosome subunit. In the mto1^Pro622∗ strain, several bands, particularly Cox1, Cox2, and Cob, are virtually missing, and the overall pattern is similar to that of the (empty) vector. In the mto1^Ala431Thr, the intensity of some bands, including Var1 and Atp6, is slightly reduced compared to the MTO1^WT strain, but the two patterns are similar, suggesting mild impairment.

Respiratory Phenotypes of Yeast Mutant Strains
(A and B) Respiratory activity of yeast mto1Δ strains transformed with MTO1^WT recombinant vector, (empty) vector, and mto1^Ala431Thrand mto1^Pro622∗ recombinant vectors at 28°C (A) and 37°C (B). Respiratory rates were normalized to the WT strain, for which the respiratory rate was 38.2 nmol min⁻¹ mg⁻¹ at 28°C and 28.9 nmol min⁻¹ mg⁻¹ at 37°C. Values are the mean of at least three independent experiments. Two-tail, unpaired t test was applied for statistical significance. ^∗: p < 0.05; ^∗∗p < 0.01; ^∗∗∗: p < 0.001. Data are represented as mean ± SD.

P^R and P^S Yeast Phenotypes
(A) Spot assay of mto1Δ P^R and P^S strains, transformed with MTO1^WT recombinant vector, (empty) vector, and mto1^Ala431Thrand mto1^Pro622∗ recombinant vectors. The assay was performed by spotting decreasing concentrations of yeast cells (10⁵, 10⁴, 10³, and 10²) on a medium supplemented with either 2% glucose (upper panel) or 2% glycerol (lower panel). See text for details.
(B) Respiratory activity of yeast MTO1^WT and mutant P^R strains at 28°C. Respiratory rates were normalized to the WT strain, for which the respiratory rate was 26.6 nmol min⁻¹ mg⁻¹. Values are the mean of at least four independent experiments. Two-tail, unpaired t test was applied for statistical significance. ^∗: p < 0.05; ^∗∗p < 0.01; ^∗∗∗: p < 0.001.
Data are represented as mean ± SD.