Mitochondria from Neurospora crassa, like mammalian mitochondria, carry out rapid, energy-linked K+ uptake and H+ release in the presence of valinomycin. The maximal rate of K+ uptake was about 1.0 mumol/mg of mitochondrial protein per min and was seen at valinomycin concentrations in the range of 100 to 200 mug per mg of mitochondrial protein and at K+ concentrations of 4 mM or above. Uptake could be supported either by substrate oxidation or by adenosine 5'-triphosphate (ATP), and was inhibited in the former case by antimycin or cyanide, in the latter case by oligomycin, and in both cases by 2,4-dinitrophenol. Mitochondria from the cytochrome-deficient mutant poky carried out substrate-driven K+ uptake at reduced rates, but oligomycin-sensitive, ATP-driven K+ uptake at rates about 60% greater than those shown by wild-type mitochondria. This result is consistent with the recent finding (Mainzer and Slayman 1976) that poky contains elevated amounts of oligomycin-sensitive mitochondrial adenosine 5'-triphosphatase activity.

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