AbstractOxidation of cytochrome c-553 and cytochrome f following single-turnover flashes was measured in isolated heterocysts. Half-times of oxidation for these two cytochromes were determined by measuring the rate of bleaching at 554 nm in heterocysts possessing cytochrome c-553, or in heterocysts in which cytochrome c-553 was replaced by plastocyanin. Flash-induced difference spectra demonstrate that a fast bleaching (t0.5 = 20–35 μs) corresponds to the oxidation of cytochrome c-553, and a slower bleaching (t0.5 = 87 μs) corresponds to the oxidation of cytochrome f
