The primary quinone acceptor and the membrane-bound c-type cytochromes of the halophilic purple nonsulfur bacterium Rhodospirillum salinarum: A spectroscopic and thermodynamic study

The mid-point potential (Em(7.0)) of the primary quinone acceptor (Qa) and the biochemical features (Em(7.0) and apparent molecular mass, MM) of the m embrane bound c-type cytochromes (cyt) involved in photosynthetic electron transfer of the halophilic phototrophic bacterium Rhodospirillum (Rs.) sali narum were determined. A tetrahemic RC bound cytochrome was found (MM of 39 .8 kDa) with Em(7.0) of the hemes equal to +304, +98, +21, -134 (+/- 8) mV as determined by dark equilibrium redox titrations in the isolated purified form. The highest potential heme (Em(7.0) = +304 mV, alpha band at 556 nm) was able to reduce the photo-oxidized reaction center (P+) in a sub-millis econd (less than or equal to 20 mu s) time scale reaction, acting most like ly as the direct electron donor to P+. The midpoint potential of the primar y electron donor (Em(7.0) = + 455 mV) was found to be close to that reporte d for the primary donor of the non-halophilic Rhodospirillum species Rs. ru brum, whereas the quinone primary electron acceptor (Qa) was different show ing the spectral features of a menaquinone molecule with Em(7.0) at -128 (/- 5) mV. A membrane bound c-type heme with Em(7.0) of 259 (+/- 1) and MM o f 40 kDa was also isolated and referred to an orthodox cytochrome c(1). The present data on the photosynthetic apparatus, along with the previous resu lts on the respiratory system [Moschettini et al. (1997) Arch Microbiol 168 : 302-309], suggest that Rs. salinarum is biochemically distinct from Rs. r ubrum, the most representative specie of the genus.