Amino acid sequences of cytochromes c-551 from the halophilic purple phototrophic bacteria, Ectothiorhodospira halophila and E. halochloris.

The cytochromes c-551 from Ectothiorhodospira halophila and E. halochloris contain 78 and 79 residues, respectively. The sequences can be aligned without the need to postulate any internal deletions or insertions to give 63% identity. They are apparently distantly related to the class I cytochromes c, based on the location of the heme attachment site near the N-terminus and the sixth ligand methionine near the C-terminus. Alignment with cytochromes c5 from Azotobacter and Pseudomonas, with cytochromes c6 from cyanobacteria and algae, and with cytochromes c-555 from the green phototrophic bacteria suggests that residues which occupy important positions in the three-dimensional structures of these proteins have their equivalents in the Ectothiorhodospira cytochromes c-551, but the levels of overall identity are very low, around 30%. Although the Ectothiorhodospira cytochromes c-551 are apparently distantly related to the above, they should be regarded as representative of a new subclass of type I bacterial cytochromes c. Homologs of all of the cytochromes c normally found in Pseudomonas and Azotobacter have now been found in one or more purple bacterial species. Among these, cytochrome c5 homologs are the most widely occurring in purple, green, and cyanobacteria. For the first time, all families of phototrophic bacteria plus Pseudomonas can be related to one another at the molecular level.