Characterization of the polypeptides of core light-harvesting complex from purple-sulfur bacteria

Abstract

Although the core light-harvesting complexes (LH1) from many purple-nonsulfur bacteria have been well characterised both biochemically and biophysically, little information is available on the properties of LH1 from purple-sulfur photosynthetic organisms. We present here the results of characterization of LH1 polypeptides isolated from two purple-sulfur bacteria, Chromatium(Chr.) tepidum and Chr. vinosum. Native LH1 complexes were extracted and purified as a reaction-centre(RC) associated form with the Qy absorption at 914 nm and 880 nm for Chr. tepidum and Chr. vinosum, respectively. Beside a subunit form of LH1 with Qy at 820 nm can be obtained for Chr. tepidum by addition of octyl-glucoside, an intermediate species with Qy around 850 nm was observed with addition of cetyl trimethylammonium bromide. Three elution peaks were observed from reserve-phase HPLC for the LH1 apopolypeptides of Chr. tepidum and two of them were identified as a- and b-polypeptides, respectively. A third component is probably a methionine-oxidized species of a-polypeptide. N-terminal amino acid of b-polypeptide is found to be methylated. Reconstitution ability to form the subunit was examined using bacteriochlorophyll a and separated a- and b-polypeptides from Chr. tepidum and purple-nonsulfur bacterium Rhodospirillum (R.) rubrum. The b-polypeptide from Chr. tepidum was found to be capable of forming uniform subunit not only with the a-polypeptide of Chr. tepidum but also with the a-polypeptide of R. rubrum.