SOLUBLE CYTOCHROMES AND A PHOTOACTIVE YELLOW PROTEIN ISOLATED FROM THE MODERATELY HALOPHILIC PURPLE PHOTOTROPHIC BACTERIUM, RHODOSPIRILLUM-SALEXIGENS

Three soluble cytochromes were found in two strains of the halophilic non-sulfur purple bacterium Rhodospirillum salexigens. These are cytochromes c2, c′ and c-551. Cytochrome c2 was recognized by the presence of positive charge at the site of electron transfer (measured by laser flash photolysis), although the protein has an overall negative charge (pI = 4.7). Cytochrome c2 has a high redox potential (300 mV) and is monomeric (13 kDa). Cytochrome c′ was recognized from its characteristic absorption spectrum. It has a redox potential of 95 mV, an isoelectric point of 4.3, and is isolated as a dimer (33 kDa) of identical subunits (14 kDa), a property which is typical of this family of proteins. R. salexigens cytochrome c-551 has an absorption spectrum similar to the low redox potential Rb. sphaeroides cytochrome c-551.5. It also has a low redox potential (-170 mV), is very acidic (pI = 4.5), and is monomeric (9 kDa), apparently containing 1 heme per protein. The existence of abundant membrane-bound cytochromes c-558 and c-551 which are approximately half reduced by ascorbate and completely reduced by dithionite suggests the presence of a tetraheme reaction center cytochrome in R. salexigens, although reaction centers purified in a previous study (Wacker et al., Biochim. Biophys. Acta (1988) 933, 299-305) did not contain a cytochrome. The most interesting observation is that R. salexigens contains a photoactive yellow protein (PYP), previously observed only in the extremely halophilic purple sulfur bacterium Ectothiorhodospira halophila. The R. salexigens PYP appears to be slightly larger than that of Ec. halophila (16 kDa vs. 14 kDa). Otherwise, these two yellow proteins have similar absorption spectra, chromatographic properties and kinetics of photobleaching and recovery. © 1990.