CsmA Protein is Associated with BChl a in the Baseplate Subantenna of Chlorosomes of the Photosynthetic Green Filamentous Bacterium Oscillochloris trichoides belonging to the Family Oscillochloridaceae
The baseplate subantenna in chlorosomes of green anoxygenic photosynthetic bacteria, belonging to the families Chloroflexaceae and Chlorobiaceae, is known to represent a complex of bacteriochlorophyll (BChl) a with the ~6 kDa CsmA proteins. Earlier, we showed the existence of a similar BChl a subantenna in chlorosomes of the photosynthetic green bacterium Oscillochloris trichoides, member of Oscillochloridaceae, the third family of green photosynthetic bacteria. However, this BChl a subantenna was not visually identified in absorption spectra of isolated Osc. trichoides chlorosomes in contrast to those of Chloroflexaceae and Chlorobiaceae. In this work, using room and low-temperature absorbance and fluorescence spectroscopy and sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis of alkaline-treated and untreated chlorosomes of Osc. trichoides, we showed that the baseplate BChl a subantenna does exist in Oscillochloridaceae chlorosomes as a complex of BChl a with the 5.7 kDa CsmA protein. The present results support the idea that the baseplate subantenna, representing a complex of BChl a with a ~6 kDa CsmA protein, is a universal interface between the BChl c subantenna of chlorosomes and the nearest light-harvesting BChl a subantenna in all three known families of green anoxygenic photosynthetic bacteria. 1. Introduction Green anoxygenic bacteria comprise three phylogenetically unrelated families of photosynthetic bacteria: green sulfur bacteria (family Chlorobiaceae) and green filamentous bacteria (families Chloroflexaceae and Oscillochloridaceae) [1–3]. In 2000, the genus Oscillochloris was excluded from the family Chloroflexaceae, and a new family Oscillochloridaceae was proposed based on phylogenetic data and unique physiological, biochemical, and chemotaxonomical properties [3]. The photosynthetic apparatus of green anoxygenic bacteria has a particular molecular organization and contains chlorosomes, unique extramembrane light-harvesting antennae structures [4, 5]. The group of chlorosome-containing bacteria was enlarged by the recently discovered new phototrophic chlorosome-containing organism Candidatus Chloracidobacterium thermophilum from the phylum Acidobacteria [6], and it is a surprising fact. Chlorosomes are ellipsoid oblong bodies of about 70–260？nm long, 30–100？nm wide, and 10–30？nm thick (depending on the species) attached to the inner surface of the cytoplasmic membrane. They are enveloped by a protein-lipid monolayer of 2-3？nm width [7]. The bulk of light-harvesting pigments (including various types of
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