A new bacteriochlorophyll a-protein complex associated with chlorosomes of green sulfur bacteria.

Chlorosomes were prepared from Chlorobium limicola f. thiosulfatophilum by sucrose density gradient centrifugation. Cells broken in the presence of 2 M NaSCN yielded three chlorosome fractions in the gradient: low density (no sucrose), medium density (approx. 18% sucrose), and high density (approx. 26% sucrose). All fractions were stable at any chlorosome concentration. Cells broken in the absence of 2 M NaSCN also yielded three fractions, but only the high-density fraction contained stable chlorosomes. The medium-density chlorosomes were stable only when highly concentrated. Upon dilution, bacteriochlorophyll (BChl) c was degraded to bacteriopheophytin c and concomitantly a band at 794 nm (BChl a) was revealed. Two 794-nm fractions were observed with the same densities as low- and medium-density chlorosomes. The protein composition of the 794-nm fractions was similar to that of the stable chlorosome fractions. All showed a 4-5 kDa (Mr) protein as a major component, but no trace of the 40-kDa protein characteristic of the water-soluble BChl a-protein of green sulfur bacteria. BChl a was present in all types of chlorosomes, in stable chlorosomes the BChl c/BChl a ratio was approx. 90. A special BChl a-protein (794 nm) inside the chlorosome is postulated to mediate energy transfer from BChl c to the water-soluble BChl a-protein in the baseplate.