PHOTOSYSTEM-I AND ITS BACTERIAL COUNTERPARTS

Photosynthetic reaction centres of green sulphur bacteria and of helio bacteria show a remarkable similarity to photosystem I of O2-evolving photosynthesis. Three features are common to this 'reaction centre I-t ype'. (1) A redox potential negative enough to reduce ferredoxin is ge nerated. (2) Iron-sulphur centres are constituents of the bound electr on acceptor complex. (3) A dimer of large, very hydrophobic protein su bunits not only binds the redox centres that are involved in the initi al steps of charge separation, but also binds the pigments of the inne r light antenna. This protein dimer is a heterodimer in photosystem I, but appears to be a homodimer in reaction centres of green sulphur ba cteria and of heliobacteria. The dimer-forming proteins contain a high ly conserved dodecapeptide to which one of the iron-sulphur centres is bound.