Polyadenylation of mutant tRNALeu(UUR) is induced by doxycycline, not EtBr. Northern blots of mitochondrial tRNAs, denoted by their decoding specificities, in A549 or GT cybrid cells as indicated: (A) cells treated with the indicated doses of doxycycline (DOX) with (+) or without (–) concomitant treatment with 5 μg/ml EtBr, for the times shown (plus 1 h of preincubation with DOX where applicable), or (B) in cells treated for the indicated times with 5 μg/ml EtBr. Control treatment with an equivalent volume of 70% ethanol, the solvent used for delivery of doxycycline, denoted by e. Note that control A549 cells are wild-type for tRNALeu(UUR), whereas GT cells are homoplasmic for the A3243G mutant tRNALeu(UUR). See Figure S6A for longer exposure of the tRNALeu(UUR) blot from GT cells. Note that the steady-state level of tRNALeu(UUR) in GT cells compared with control cells is low, due to the A3243G mutation, as seen in part (B). Thus, a much longer exposure is shown for the tRNALeu(UUR) panel than for the tRNASer(UCN) panel in part (A).