GENES ENCODING 2 CHLOROSOME COMPONENTS FROM THE GREEN SULFUR BACTERIA CHLOROBIUM-VIBRIOFORME STRAIN 83271D AND CHLOROBIUM-TEPIDUM

Chlorosomes of the thermophilic green sulfur bacterium Chlorobium tepidum have been isolated and their polypeptides analyzed by polyacrylamide gel electrophoresis and amino acid sequencing. These chlorosomes were shown to contain nine different polypeptides ranging in mass from approximately 6 to 27 kDa. The csmA gene, encoding a highly abundant chlorosome protein with a mass of 6.2 kDa, were cloned and sequenced from both Chlorobium vibrioforme strain 8327D and Chlorobium tepidum. The gene from both species predicts identical proteins of 79 amino acid residues, and a comparison of the deduced sequence with that determined for the protein indicates that 20 amino acid residues are post-translationally removed from the carboxyl-terminus of the CsmA precursor. Transcript analyses showed that in Chlorobium tepidum the csmA gene is encoded on two transcripts of approximately 350 and 940 nucleotides; the smaller transcript probably results from processing of the larger RNA molecule. Transcription of the longer mRNA initiates 68 basepairs upstream from the start codon of a second open reading frame that is located 154 nucleotides 5' to csmA and that predicts a protein of 139 amino acid residues. The amino-terminal sequence determined for a 14.5 kDa polypeptide in the chlorosomes of Chlorobium tepidum matched the sequence deduced from this open reading frame except for the absence of the initiator methionine residue; accordingly, this gene has been named csmC. A comparison of the genomic organization of the csmA loci in Chlorobium vibrioforme, Chlorobium tepidum, and Chloroflexus aurantiacus were found to be surprisingly similar.