Pentamidine inhibits in vitro splicing of nuclear
group I introns from rRNA genes of some pathogenic fungi
and is known to inhibit mitochondrial function in yeast.
Here we report that pentamidine inhibits the self-splicing
of three group I and two group II introns of yeast mitochondria.
Comparison of yeast strains with different configurations
of mitochondrial introns (12, 5, 4, or 0 introns) revealed
that strains with the most introns were the most sensitive
to growth inhibition by pentamidine on glycerol medium.
Analysis of blots of RNA from yeast strains grown in raffinose
medium in the presence or absence of pentamidine revealed
that the splicing of seven group I and two group II introns
that have intron reading frames was inhibited by the drug
to varying extents. Three introns without reading frames
were unaffected by the drug in vivo, and two of these were
inhibited in vitro, implying that the drug affects splicing
by acting directly on RNA in vitro, but on another target
in vivo. Because the most sensitive introns in vivo are
the ones whose splicing depends on a maturase encoded by
the intron reading frames, we tested pentamidine for effects
on mitochondrial translation. We found that the drug inhibits
mitochondrial but not cytoplasmic translation in cells
at concentrations that inhibit mitochondrial intron splicing.
Therefore, pentamidine is a potent and specific inhibitor
of mitochondrial translation, and this effect explains
most or all of its effects on respiratory growth and on
in vivo splicing of mitochondrial introns.