Electron Transfer Around Photosystem I in Cyanobacterial Heterocyst Membranes

Abstract

Cyanobacteria are unique amongst the prokaryotes in possessing a higher plant—type of photosynthesis, with two photosystems linked in series. The heterocyst is a specialized cell type occurring in some filamentous strains at a frequency of 5–10%, and is the site of N ₂ — fixation under aerobic conditions. During differentiation of the heterocyst, the O ₂ -evolving PSII is lost and cyclic electron transfer around PSI predomi nates. The absence of PSII reaction centres and the diminished levels of accessory pigments give membranes isolated from heterocysts excellent properties for spectroscopic studies. Soluble components such as plastocyanin, cytochrome c -553 and PSI acceptors, washed from the membranes during isolation, may be selectively reconstituted. Additionally, the presence of an endogenous uptake hydrogenase which can be utilized experimentally to reduce the plastoquinone pool and the electron transfer chain, make heterocyst membranes a useful system in which to study cyclic electron flow.