Live-cell pri-miRNA processing assay shows that Co(III)PPIX activates pri-miRNA processing without inducing cytotoxicity. Hela cells were cultured in heme-depleted media, transfected with the pri-miR-9-1 reporter, treated for 10 h with succinylacetone (1 mM) either alone or together with Co(III)PPIX or hemin at the indicated concentrations. (A) Normalized eYFP/mCherry fluorescence slopes (± 95% CI). (B) Abundance of mature miR-9 normalized by that of β-actin mRNA (mean ± SD, n = 4). (C) MTT assays showed little cytotoxicity (mean ± SD, n = 4). (D–F) Reporter assays were performed similarly to described above, except that the N-flag-DGCR8 expression plasmid was cotransfected with the reporters, that the pri-miR-9-1 (D), pri-miR-185 (E), and pri-miR-30a (F) reporters were used, and that SA, Co(III)PPIX and hemin were added prior to transfection. See also Figure S4.