THE CYTOCHROME SUBUNIT STRUCTURE IN THE PHOTOSYNTHETIC REACTION-CENTER OF CHROMATIUM-MINUTISSIMUM

Gel-electrophoretic assay revealed that the photosynthetic reaction ce nter (RC) of Chromatium minutissimum, in contrast to the well-known RC Rhodopseudomonas viridis, consists of five rather than four subunits with molecular masses of 37, 34, 25, 19, and 17 kDa. The 37- and 19-kD a subunits are stained with tetramethylbenzidine for the cytochrome c hemes. Absorption spectra show that the concentration of reduced cytoc hromes in the C. minutissimum RC poised at redox potential of -150 mV (fully reduced pool of hemes) is about three times more than in the C. minutissimum RC poised at redox potential of +260 mV (only high-poten tial hemes are reduced). The results of redox titration of absorption changes at the cytochrome c alpha-band are most appropriately approxim ated by a six-component theoretical curve with the midpoint potentials of E-m1 = 390 mV, E-m2 = 320 mV, E-m3 = 210 mV, E-m4 = 100 mV, E-m5 = 20 mV, and E-m6 = -50 mV. Possible functions of the cytochromes with the midpoint potentials 210 and 100 mV, which have not been found in p urple bacteria before, are discussed. (C) 1998 Federation of European Biochemical Societies.