Pigment-protein diversity in chlorosomes of green phototrophic bacteria

Abstract

In order to compare and contrast the structure and function of the light-harvesting antennae (i.e. chlorosomes) of green bacteria, a procedure for isolating and characterizing them from green sulfur bacteria was developed. The chlorosomes from Chlorobium species with bacteriochlorophyll (Bchl) c or e were isolated by a two step sucrose density centrifugation in the presence of 2% miranol, a mild detergent, and 2 M sodium thiocyanate (NaSCN). Purified chlorosomes from two green sulfur bacteria, Chlorobium phaeobacteroides and Chlorobium tepidum, and the filamentous green bacterium Chloroflexus aurantiacus were analysed by spectrophotometry, SDS-polyacrylamide gel electrophoresis, and immunological procedures. Isolated chlorosomes from both Chlorobium species contain only two electrophoretically separable protein components with approximate molecular masses of 5–7.5 and 34.5 kDa. In addition, they have a major light-harvesting antenna pigment (Bchl c or e), a minor Bchl a species, and carotenoids. Chloroflexus aurantiacus antisera for the three major chlorosome proteins (5.6, 11, and 18 kDa), and the reaction center proteins (24 and 24.5 kDa) did not cross react with any Chlorobium proteins analyzed in this study. Chlorobium limicola f. thiosulfatophilum antisera against the 7.5 kDa chlorosome protein cross reacted strongly with the 5–7.5 kDa protein from Cb. tepidum, weakly with the Cb. phaeobacteroides protein, but not at all to the 5.6 kDa chlorosome protein from Cf. aurantiacus. These results provide further evidence for the evolutionary divergence of the chlorosomes from green phototrophic bacteria (e.g., Chlorobium-type and Chloroflexus-type).