Electron transfer may occur in the chlorosome envelope: The CsmI and CsmJ proteins of chlorosomes are 2Fe-2S ferrodoxins

Chlorosomes of the green sulfur bacterium Chlorobium tepidium have previous ly been shown to contain at least 10 polypeptides [Chung, S., Frank, G., Zu ber, H,, and Bryant, D. A. (1994) Photosynth. Res. 31, 261-275]. Based upon the N-terminal amino acid sequences determined for two of these proteins, the corresponding genes were isolated using degenerate oligonucleotide hybr idization probes. The csmI and csmJ genes encode proteins of 244 and 225 am ino acids, respectively. A third gene, denoted csmX, that predicts a protei n of 221 amino acids with strong sequence similarity to CsmI and CsmJ, was found to be encoded immediately upstream from the csmJ gene, All three prot eins have strong sequence similarity in their amino-terminal domains to [2F e-2S] ferredoxins of the adrenodoxin/putidaredoxin subfamily of ferredoxins . CsmI and CsmJ were overproduced in Escherichia coli, and both proteins we re shown by EPR spectroscopy to contain iron-sulfur clusters. The g-tensor and relaxation properties are consistent with 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 pr oteins. Redox titration of isolated chlorosomes showed these clusters to ha ve potentials of about -201 and +92 mV vs SPIE. The former potential is sim ilar to that measured by redo,tt. titration of the clusters in inclusion bo dies of CsmJ. Possible roles for these iron-sulfur proteins in electron tra nsport and light harvesting are discussed.