Sequence evidence for strong conservation of the photoactive yellow proteins from the halophilic phototrophic bacteria Chromatium salexigens and Rhodospirillum salexigens

The photoactive yellow proteins (PYP) have been found to date only in three species of halophilicpurple phototrophicbacteria. They have photochemical activity remarkably similar to that of the bacterial rhodopsins. In contrast to rhodopsins, however, the PYPs are small water-soluble proteins. We now report the complete amino acid sequences of Rhodospirillum salexigens and Chromatiumsalexigens PYP which allow comparison with the known sequence and three-dimensional structure of the prototypic protein from Ectothiorhodospira halophila. Although isolated from three different families of bacteria, the PYP sequences are 70−76% identical. All three contain 125 amino acid residues, and no insertions or deletions are necessary for alignment. This is a remarkable result when it is considered that electron transfer proteins from these purple bacterial species are only 25−40% identical and that insertions and deletions are needed for their proper alignment. It thus appears that PYP has the same important function...