EVOLUTION OF HELIOBACTERIA - IMPLICATIONS FOR PHOTOSYNTHETIC REACTION-CENTER COMPLEXES

The evolutionary position of the heliobacteria, a group of green photo synthetic bacteria with a photosynthetic apparatus functionally resemb ling Photosystem I of plants and cyanobacteria, has been investigated with respect to the evolutionary relationship to Gram-positive bacteri a and cyanobacteria. On the basis of 16S rRNA sequence analysis, the h eliobacteria appear to be most closely related to Gram-positive bacter ia, but also an evolutionary link to cyanobacteria is evident. Interes tingly, a 46-residue domain including the putative sixth membrane-span ning region of the heliobacterial reaction center protein shows rather strong similarity (33% identity and 72% similarity) to a region inclu ding the sixth membrane-spanning region of the CP47 protein, a chlorop hyll-binding core antenna polypeptide of Photosystem II. The N-termina l half of the heliobacterial reaction center polypeptide shows a moder ate sequence similarity (22% identity over 232 residues) with the CP47 protein, which is significantly more than the similarity with the Pho tosystem I core polypeptides in this region. An evolutionary model for photosynthetic reaction center complexes is discussed, in which an an cestral homodimeric reaction center protein (possibly resembling the h eliobacterial reaction center protein) with 11 membrane-spanning regio ns per polypeptide has diverged to give rise to the core of Photosyste m I, Photosystem II, and of the photosynthetic apparatus in green, pur ple, and heliobacteria.