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.

OPTIMAL SPECTRAL COORDINATION OF SUBANTENNAE IN NATURAL ANTENNAE AS AN EFFICIENT STRATEGY FOR LIGHT HARVESTING IN PHOTOSYNTHESIS

This work continues a series of our investigations on efficient strategies of functioning of natural light-harvesting antennae, initiated by a concept of rigorous optimization of photosynthetic apparatus by functional criterion, and deals with the problem of an optimal spectral coordination of subantennae in photosynthetic superantenna of the green bacterium Oscillochloris trichoides from a new family of green bacteria Oscillochloridaceae based in 2000. At present, two subantennae were identified surely: chlorosomal BChl c subantenna B750 and membrane BChl a subantennae B805-860. Some indirect experiments indicated on the presence of minor amounts of BChl a in isolated chlorosomes which allowed us to propose on the existence of an intermediate-energy subantenna which can connect the chlorosomal BChl c and the membrane BChl a ones. However, in the absorption spectra of isolated chlorosomes, this BChl a subantenna was not visually identified. This promoted us to perform a theoretical analysis of the optimality of spectral coordination of Oscillochloris trichoides subantennae. Using mathematical modeling for the functioning of the natural superantenna, we showed that an intermediate-energy subantenna, connecting B750 and B805-860 ones, allows one to control superantenna efficiency, i.e. to optimize the excitation energy transfer from B750 to B805 by functional criterion, and hence, the existence of such intermediate-energy subantenna is biologically expedient.