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The amino acid sequences of the reaction center-bound cytochrome subunit of six species of purple bacteria were compared. Amino acid residues thought to be important in controlling the redox midpoint potentials of four hemes in Blastochloris (Rhodopseudomonas) viridis were found to be well conserved. As opposed to all other species studied, the amino acid sequence of the cytochrome subunit of B. viridis had several insertions of more than 10 residues at specific regions close to the LM core, suggesting that interaction of the cytochrome subunit with the LM core in most species is different from that in B. viridis. Distribution of charged amino acid residues on the surface of the cytochrome subunit was compared among six species and discussed from the viewpoint of interaction with soluble electron donors.