The photosystem of heliobacteria consists of a core of homodimeric proteins that, like PSI and the photosystems of GSB, contains 11 TMH per monomer . From the homodimeric organization it is tempting to conclude that heliobacterial photosystems are closely related to the homodimeric photosystems of green sulfur bacteria (see Section 13.4). When respective sequences are compared, however, heliobacterial photosystems appear slightly more related to the photosystems of GSB (17.4% sequence identity) than to PsaA/PsaB of PSI (16.3% and 16.8% sequence similarity) found in oxygenic phototrophs -27] - However, the heliobac-trial RC shows greater similarity to the PSI (PsaA/PsaB) of oxygenic phototrophs, and parts of the core antennae show remarkable similarities to the core antennae protein CP47 of PSII, which contains a Q-type RC. . Interestingly other phylo-genetic indicators also assign cyanobacteria as the closest photosynthetic relatives of heliobacteria . This position - having the simplest of all photosynthetic RCs in composition, but being closely related to organisms with the most diversified photosynthetic apparatus -makes the evolution of the heliobactrial RC a much discussed topic. The homodimeric photosystem proteins, and their overall weak identity to either PsaA or PsaB, implies that the green sulfur bacterial photosystem split before the gene duplication that lead to PsaA/ PsaB of PSI. Is the heliobactrial photosystem a living fossil that closely resembles the Ur-RC? One sign that this may be the case is that heliobacteria did not acquire an external light-harvesting system, making heliobacteria the only photosynthetic group of organisms that has not developed pigmented proteins to extend the absorption cross section of the RC. Heliobacteria seem to prefer heterotrophic conditions and are not known to be able to perform inorganic carbon fixation.
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