Abstract: | The rate of electron transfer through Photosystem I (reduced 2,6-dichlorophenol indophenol (DCIPH2 → methylviologen) in a low-salt thylakoid suspension is inhibited by Mg2+ both under light-limited and the light-saturated conditions, the magnitude of inhibition being the same. The 2,6-dichlorophenol indophenol (DCIP) concentration dependence of the light-saturated rate in the presence and in the absence of Mg2+ shows that the overall rate constant of the photoreaction is not altered by Mg2+. With N,N,N′,N′-tetramethyl-p-phenylenediamine or 2,3,5,6-tetramethylphenylenediamine as electron donor only the light-limited rate, not the light-saturated rate, is inhibited by Mg2+ and the magnitude of inhibition is the same as with DCIP as donor. The results are interpreted in terms of heterogeneous Photosystem I, consisting of two types, PS I-A and PS I-B, where PS I-A is involved in cation-regulation of excitation energy distribution and becomes unavailable for DCIPH2 → methyl viologen photoelectron transfer in the presence of Mg2+. |