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 halophilic purple phototrophic bacteria. They have photochemical activity remarkably similar to that of the bacteria 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 Chromatium salexigens 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 in each of the purple bacteria and that most of the amino acid residues are necessary to maintain structure and function. By most standards, PYP would be called a "slowly evolving protein". R. salexigens PYP is uniquely degraded by proteolysis at low ionic strength, probably as a consequence of unfolding due to electrostatic repulsion of the excess negative charge. Therefore it may also be classified as a "halophilic protein".