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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.

154This work continues the cycle of studies of the strategy of efficient functioning of natural light har vesting antennae, in particular, inhomogeneous super antennae of green bacteria, consisting of several homogeneous subantennae [1]. In our previous paper [2], mathematical modeling of the dynamics of excita tion energy transfer in inhomogeneous superantenna of photosynthetic green bacteria Oscillochloris (Osc.) trichoides (using formalism of probability matrices) allowed us to determine the theoretically optimal spectral composition of homogeneous superantennae of Osc. trichoides, which showed the importance of optimal interfacing of their energy levels for efficient and stable functioning of the entire superantenna as a whole. In particular, we theoretically substantiated the necessity of existence in the Osc. trichoides superan tenna of an intermediate BChl a subantenna with the energy level in the range of ~790-800 nm for optimiz ing energy transfer from the chlorosome B750 suban tenna to the membrane B805 subantenna.This paper presents an experimental proof of the validity of the theoretical inferences made in our pre vious work.The light harvesting superantenna of Osc. tri choides is located in separate subcellular structures: the chlorosome containing the oligomeric BChl c sub antenna B750 and the cytoplasmic membrane con taining the BChl a subantenna B805-860, which is an analogue of the membrane subantenna B808-866 of Chloroflecsus aurantiacus [3].Electron microscopic analysis of Osc. trichoides cells thin sections showed that the chlorosome and membrane subantennae are separated by a lamellar structure, which is similar to the base plates in green bacteria of two other families, Chloroflexaceae and Chlorobiaceae, which contain BChl a subantenna coupling the chlorosome Bchl c/d/e and, respectively, membrane BChl a or FMO subantennae [4]. How ever, the BChl a component was absent in the absorp tion spectra of isolated Osc. trichoides chlorosomes, unlike the chlorosomes of two other families. Never theless, indirect data suggest the presence in isolated Osc. trichoides chlorosomes of negligible amounts of BChl a, which, however, may be a residual BChl a of the subantenna B805-860. In turn, generalized theo retical analysis of the optimality of the spectral com position of the superantenna of Osc. trichoides clearly demonstrated the biological expedience of the exist ence of an intermediate BChl a subantenna (with the energy level position in the range of 790-800 nm), coupling the chlorosome BChl c subantenna B750 to the membrane BChl a subantenna B805-860, which provides stability and high efficiency of functioning of the entire superantenna as a whole. This allowed us to assume that the presence in isolated Osc. trichoides chlorosomes of negligible amounts of BChl a, which was shown in our previous work, indicates that the hypothetical BChl a subantenna located in the base plate of Osc. trichoides exists in vivo.To confirm this assumption, we performed five series of experiments using identical chloros...

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