Genes encoding two chlorosome components from the green sulfur bacteriaChlorobium vibrioforme strain 8327D...

Chlorosomes of the thermophilic green sulfur bacteriumChlorobium 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. ThecsmA gene, encoding a highly abundant chlorosome protein with a mass of 6.2 kDa, were cloned and sequenced from bothChlorobium vibrioforme strain 8327D andChlorobium 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 inChlorobium tepidum thecsmA 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' tocsmA and that predicts a protein of 139 amino acid residues. The amino-terminal sequence determined for a 14.5 kDa polypeptide in the chlorosomes ofChlorobium tepidum matched the sequence deduced from this open reading frame except for the absence of the initiator methionine residue; accordingly, this gene has been namedcsmC. A comparison of the genomic organization of thecsmA loci inChlorobium vibrioforme, Chlorobium tepidum, andChloroflexus aurantiacus were found to be surprisingly similar.