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 ph otochemical activity remarkably similar to that of the bacterial rhodo psins. In contrast to rhodopsins, however, the PYPs are small water-so luble proteins, We now report the complete amino acid sequences of Rho dospirillum salexigens and Chromatium salexigens PYP which allow compa rison with the known sequence and three-dimensional structure of the p rototypic protein from Ectothiorhodospira halophila. Although isolated from three different families of bacteria, the PYP sequences are 70-7 6% identical. All three contain 125 amino acid residues, and no insert ions or deletions are necessary for alignment, This is a remarkable re sult 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 bacteri a and that most of the amino acid residues are necessary to maintain s tructure and function. By most standards, PYP would be called a ''slow ly evolving protein''. R. salexigens PYP is uniquely degraded by prote olysis at low ionic strength, probably as a consequence of unfolding d ue to electrostatic repulsion of the excess negative charge. Therefore it may also be classified as a ''halophilic protein''