Biochemical and spectral characterization of the core light harvesting complex 1 (LH1) from the thermophilic purple sulfur bacterium Chromatium tepidum

Abstract

The photoreceptor complex (LH1-RC) of the thermophilic purple sulfur bacterium Chromatium tepidum (Chr. tepidum) has been investigated and isolated using the detergent Triton X-100 followed by sucrose density gradient centrifugation (SDGC). The isolated LH1-RC complex had an absorption maximum at 915 nm, the longest absorption band observed in light harvesting complex 1 (LH1) in purple bacteria containing bacteriochlorophyll a (BChl a). The absorption maximum of this LH1-RC complex showed a 30 nm blue-shift to 885 nm after being purified by DEAE cellulose column chromatography. The blue-shifted absorption band was found to return to 915 nm after removing the salt ion, indicating a reversible effect of salt ion on this LH1-RC complex. We have determined the nucleotide sequences of genes coding for the α, β polypeptide subunits of the LH1 core complex from Chr. tepidum and have found some characteristic features. The deduced amino acid sequences of the α subunit showed a deletion of an arginine residue in the C terminal of membrane helix. A possible correlation between the unusual absorption behavior and structural features of α and β polypeptide subunits of the LH1 core complex is discussed.