Two molecules of cytochrome c function as the electron donors to P840 in the reaction center complex isolated from a green sulfur bacterium, Chlorobium tepidum

A photoactive reaction center complex was isolated from a thermophilic green sulfur bacterium, Chlorobium tepidum under anaerobic conditions. The electron transfer occurred from heme c to the photo-oxidized reaction center chlorophyll, P840+, with a half time (t1/2) of 110 or 340 microseconds at 24 or 12 degrees C, respectively. Optical measurements under multiflash excitations indicated that two hemes function as the immediate electron donors to P840+. SDS-PAGE analysis of the RC complex in combination with the N-terminal amino acid sequence analyses revealed five subunit bands; a core protein (65 kDa), the light harvesting bacteriochlorophyll alpha protein (41 kDa), a protein with 2[4Fe-4S] clusters (31 kDa), monoheme cytochrome c (22 kDa), and a 18-kDa protein whose function is unknown. The reaction center complex, thus, contains two molecules of cytochrome c per P840.