The antenna complexes of photosynthetic bacteria function as light-harvesting systems that absorb light and transfer the excitation energyto the reaction centres. The antenna complexes usually comprise 2polypeptides (alpha- and beta-chains), 2-3 bacteriochlorophyll moleculesand some carotenoids [<cite idref="PUB00001417"/>, <cite idref="PUB00003468"/>].The alpha- and beta-chains are small proteins of 40-70 residues. Each has an N-terminal hydrophilic cytoplasmic domain, a single transmembrane (TM)region, and a small C-terminal hydrophilic periplasmic domain. In bothchains, the TM domain houses a conserved His residue, presumed to beinvolved in binding the magnesium atom of a bacteriochlorophyll group.The beta-chains are characterised by a further histidine at the C-terminalextremity of the cytoplasmic domain, which is also thought to be involvedin bacteriochlorophyll binding.<p>This family represents the antenna pigment protein (also known as light harvesting protein B) beta chain.</p> Antenna complex, beta subunit