Bacteriochlorophyll a- and c-Protein Complexes from Chlorosomes of Green Sulfur Bacteria Compared with Bacteriochlorophyll c Aggregates in CH₂Cl₂-Hexane

Abstract

The two groups of green bacteria, Chlorobiaceae and Chloroflexaceae, contain bacteriochlorophyll (BChl) c or a related pigment as the main light-harvesting pigment [1]. In both groups all of the BChl c is located in chlorosomes, oblong bodies appressed to the inner surface of the cytoplasmic membrane as shown in Fig. 1. Inside the chlorosome are proteinaceous structures called rod elements which are thought to be made up of BChl c—binding proteins. These BChl c—binding proteins have recently been extracted from chlorosomes of Chloroflexus aurantiacus (Chloroflexaceae) [2] and Chlorobium limicola f. thiosulfatophilum (Chlorobiaceae). The Chloroflexus protein contains 51 amino acid residues, of which 4 are Asn and 4 are Gln. Similarly the Chlorobium protein contains ca. 55 residues, of which ca. 9 are Asn + Gln. Neither protein contains any Cys. The Chloroflexus protein contains no Lys and only one His, while the Chlorobium protein appears to contain little or no Arg, Trp, Tyr and His. From the amino acid sequence of the Chloroflexus protein it has been deduced that the protein can form an α-helix in which the amide groups (Asn and Gln) are located predominantly on one side of the helix. WECHSLER et al. [2] have proposed that 7 BChl c molecules are bound to each α-helix via interactions with 7 amide groups on the protein. Although the Chlorobium protein apparently shows little homology with the amino acid sequence of the Chloroflexus protein, we suppose that BChl c might be bound to the Chlorobium protein by a similar interaction. However, we do not yet know the complete sequence and probable conformation of the polypeptide.