<p>Green sulphur bacteria contain a photosynthetic reaction centre (PRC) complex that is analogous to the photosystem I complex found in plants. P840, a special pair of bacteriochlorophyll a molecules, functions as the primary electron donor in this complex. This entry represents a cytochrome c551 component of the PRC which can act as the immediate electron donor to P840 in vivo, though it is not known if it functions as the physiological donor [<cite idref="PUB00043527"/>, <cite idref="PUB00043528"/>, <cite idref="PUB00043529"/>].</p> Cytochrome c551