(A) Three-day proliferation assay of control or FXN KO K562 cells. Cells were grown in 21% O2, 1% O2 or 21% O2 with 75 μM FG-4592, which stabilizes HIF1a regardless of oxygen tensions. (B) Immunoblot of control or FXN KO cells grown in 21% O2, 1% O2 or 21% O2 with 75μM FG-4592, blotted for FXN, ISCU, NFS1, LYRM4, HIF1a and TIMM23. Asterisk indicates a non-specific band. (C) Three-day proliferation assay of control or FXN KO 293T cells. Cells were grown in 21% O2, 1% O2 or 21% O2 with 75μM FG-4592. (D) Growth of WT or Δyfh1 yeast in 21% O2 or 1% O2. (E) Three-day proliferation assay of control or FXN KO K562 cells in 21% O2 supplemented with DMSO or the antioxidants Mito-TEMPO (5nM,50nM,500nM), NAC (5μM,50μM,500μM) or Mn(III)TBAP (1μM,10μM,100μM). All bar plots show mean ± SD. *=p < 0.05, ***=p < 0.001, ****=p < 0.0001. One-way ANOVA with Bonferroni’s post-test. See also Figure S1.