| Abstract
| - Chlorosomes of the green sulfur bacterium Chlorobium tepidum have previously been shownto contain at least 10 polypeptides [Chung, S., Frank, G., Zuber, H., and Bryant, D. A. (1994) Photosynth.Res.41, 261−275]. Based upon the N-terminal amino acid sequences determined for two of these proteins,the corresponding genes were isolated using degenerate oligonucleotide hybridization probes. The csmIand csmJ genes encode proteins of 244 and 225 amino acids, respectively. A third gene, denoted csmX,that predicts a protein of 221 amino acids with strong sequence similarity to CsmI and CsmJ, was foundto be encoded immediately upstream from the csmJ gene. All three proteins have strong sequence similarityin their amino-terminal domains to [2Fe-2S] ferredoxins of the adrenodoxin/putidaredoxin subfamily offerredoxins. CsmI and CsmJ were overproduced in Escherichia coli, and both proteins were shown byEPR spectroscopy to contain iron−sulfur clusters. The g-tensor and relaxation properties are consistentwith their assignment as [2Fe-2S] clusters. Isolated chlorosomes were also shown to contain [2Fe-2S]clusters whose properties were similar to those of the recombinant CsmI and CsmJ proteins. Redox titrationof isolated chlorosomes showed these clusters to have potentials of about −201 and +92 mV vs SHE.The former potential is similar to that measured by redox titration of the clusters in inclusion bodies ofCsmJ. Possible roles for these iron−sulfur proteins in electron transport and light harvesting are discussed.
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