Phylogeny of the PscB reaction center protein from green sulfur bacteria.

The reaction center (RC) of green sulfur bacteria has iron-sulfur clusters as terminal acceptors and is related to the Type I RC found in Heliobacter sp. and in Photosystem I (PS I) of green plants and cyanobacteria. Degenerate primers were used to retrieve the genes coding for one of the RC proteins, PscB, from 11 strains of green sulfur bacteria. PCR using the same primers gave no product with a second group of strains and the protein from these strains did not crossreact with antibodies raised against purified PscB from the first group, suggesting the presence of a high degree of variability. The sequences shared a high degree of similarity in the region coding for the binding motif for the 4Fe-4S centers. However, the N-terminal portion of the deduced protein sequences was highly variable and contained a highly positively charged, low-complexity region with repeated tetrapeptides with two alanines flanked by proline or lysine. The PscB sequences obtained fell into two major groups, and the results suggested a lack of correlation between the pigmentation of the chlorosome antenna system and the reaction center protein. There is also a lack of correlation between the grouping of the pscB sequences and the phylogeny deduced from 16S rRNA.