Spectroscopic properties of a reconstituted light-harvesting complex from the green sulfur bacterium Chlorobium tepidum containing CsmA and bacteriochlorophyll a

Green sulfur bacteria possess two light-harvesting antenna systems, the chlorosome and the

Fenna-Matthews-Olson (FMO) protein. In addition to self-aggregated bacteriochlorophyll (BChl) c,

chlorosomes of Chlorobium tepidum contain a small amount of BChl a (ratio 100:1). The chlorosomal

BChl a is associated with CsmA, a 6.2 kDa protein that accounts for more than 50% of the protein

content of chlorosomes. This CsmA-BChl a complex is located in the chlorosome baseplate with the

hydrophilic C-terminal part of CsmA in contact with the FMO protein. CsmA was purified from Chl.

tepidum. Isolated chlorosomes were lyophilized and extracted with chloroform/methanol (1:1, v/v). The

. Isolated chlorosomes were lyophilized and extracted with chloroform/methanol (1:1, v/v). The

extract was further purified using gel filtration and reverse-phase HPLC and the purity of the preparation

confirmed by SDS-PAGE. Mass spectrometric analysis showed an m/z of 6154.8, in agreement with the

calculated mass of the csmA gene product after C-terminal processing. CD spectroscopy of the isolated

protein showed that the main structural motif was an R-helix. We have reconstituted the isolated CsmA

protein with BChl a in micelles of n-octyl â-D-glucopyranoside. The resulting preparation reproduced the

spectral characteristics of the CsmA-BChl a complex present in the chlorosome baseplate.