AMINO-ACID-SEQUENCES OF CYTOCHROMES C(2) AND C' FROM THE MODERATELY HALOPHILIC PURPLE PHOTOTROPHIC BACTERIUM RHODOSPIRILLUM SALEXIGENS

Rhodospirillum salexigens is a moderately halophilic purple phototroph ic bacterium which grows optimally in 8% NaCl. The amino acid sequence s of the two principal soluble cytochromes c have been determined. One of these is a cytochrome c(2), similar in size to mitochondrial cytoc hrome c. While clearly of the same sequence class as mitochondrial cyt ochrome c and the proteins from several other Gram-negative bacteria, it does not show particular affinity to any already known sequence in terms of the percentage sequence identity. The other protein is a cyto chrome c', but is also a divergent member of this widespread group. Th e lack of appreciable sequence identity to other species is probably d ue to a limit of divergence which has been reached for the majority of purple bacterial species. However, the numbers of insertions and dele tions and their locations in cytochromes c, and c' suggest that R sale xigens may be related to Rhodospirillum molischianum. Like other elect ron transport proteins from halophiles, both of these cytochromes are notable for their high content of arginine as compared with lysine and both are acidic. However, they do not show any particular sequence ho mology to electron transport proteins that have been characterized fro m the extremely halophilic phototrophes of the genus Ectothiorhodospir a. Thus, it appears that adaptation to halophilic habitats has indepen dently occurred more than once in purple bacteria.