Reconstitution of electron transport by cytochrome c-553 in a cell-free system of Nostoc muscorum

Treatment of spheroplasts of Nostoc museorum with hypotonic buffer results in membranes depleted of cytochrome c-553, but still active in photosynthetic and respiratory electron transport. These membranes retain full photosystem II activity (H₂O→DAD_ox). Complete linear electron transport (H₂O→NADP⁺), however, is decreased as compared with untreated spheroplasts. Addition of basic Nostoc cytochrome c-553 to depleted membranes reconstitutes NADP⁺ reduction and redox reactions of the photosystem I region as well. Using NADPH as electron donor, respiration of depleted membranes is also stimulated by adding cytochrome c-553, indicative of its function in respiratory electron transport. Cytochrome c-553 from Bumilleriopsis filiformis, Spirulina platensis (acidic types), Phormidium foveolarum (basic type), and mitochondrial horse-heart cytochrome c-550 are not effective in reconstituting both photosynthetic and respiratory electron transport, which points to a specific role of Nostoc cytochrome c-553.