Purification and characterization of the photochemical reaction center of the thermophilic purple sulfur bacterium Chromatium tepidum

Pure reaction center preparations from the thermophilic species Chromatium tepidum have been obtained by lauryldimethylamine N-oxide treatment of chromatophores. The light-induced difference spectrum in presence of 10 mM sodium ascorbate revealed the presence of two high-potential cytochrome c hemes (-band, 555 nm; γ-band, 422 nm). The dithionite-minus-oxidized difference spectrum in the -band suggests the presence of additional hemes of low potential. These hemes are associated with a single polypeptide (M_r = 36 000). The reaction center pigments, probably four bacteriochorophyll a and two bacteriopheophytin a molecules, are associated with three polypeptides of apparent molecular weights equal to 33 000, 30 000 and 22 000. A carotenoid molecule is also bound to the reaction center. The three main absorption bands of this molecule are located at 480, 510 and 530 nm at liquid helium temperature. Photochemical activity is found to be stable, even after heating for 10 min at temperatures higher than 60 °C in intact chromatophore membranes. On the other hand, isolated reaction centers or chromatophores treated with 1% lauryldimethylamine N-oxide are fully inactivated after heating at temperatures higher than 50 °C. From these results, we propose that lipid-protein interactions are of prime importance in the thermal stabilization of Chromatium tepidum reaction centers.