PHOTOSYSTEM-I AND ITS BACTERIAL COUNTERPARTS

Photosynthetic reaction centres of green sulphur bacteria and of heliobacteria show a remarkable similarity to photosystem I of O2-evolving photosynthesis. Three features are common to this 'reaction centre I-type'. (1) A redox potential negative enough to reduce ferredoxin is generated. (2) Iron-sulphur centres are constituents of the bound electron acceptor complex. (3) A dimer of large, very hydrophobic protein subunits not only binds the redox centres that are involved in the initial steps of charge separation, but also binds the pigments of the inner light antenna. This protein dimer is a heterodimer in photosystem I, but appears to be a homodimer in reaction centres of green sulphur bacteria and of heliobacteria. The dimer-forming proteins contain a highly conserved dodecapeptide to which one of the iron-sulphur centres is bound.