Gene encoding the 5.7-kilodalton chlorosome protein of Chloroflexus aurantiacus: regulated message levels and a predicted carboxy-terminal protein extension.

The major light-harvesting pigment of the green filamentous bacterium Chloroflexus aurantiacus is bacteriochlorophyll (Bchl) c, localized in chlorosomes attached to the inner surface of the cytoplasmic membrane. Chlorosomes consist of four polypeptides and associated pigments and lipids. Previous studies of the inducible assembly of the photosynthetic apparatus had indicated that the major chlorosomal polypeptides are present as high-molecular-weight aggregates before the appearance of mature chlorosomes, and a mechanism for posttranslational processing of a polyprotein had been proposed. We have isolated the gene (csmA) encoding the 5.7-kilodalton chlorosomal polypeptide from C. aurantiacus in order to determine whether this protein is synthesized as part of a polyprotein. Analysis of the nucleotide sequence of csmA indicates that the gene is not large enough to encode more than one known chlorosome polypeptide. Transcriptional analysis indicates that csmA is transcribed as a small message whose abundance is regulated in response to oxygen, so that no csmA message is detectable in cells grown aerobically in the dark. Comparison of the sequence predicted by csmA with the peptide sequence of the Bchl c binding protein purified from chlorosomes indicates that this protein is synthesized with a carboxy-terminal extension of 27 amino acids. We discuss possible roles for this carboxy-terminal extension in the assembly of chlorosomes.