The electron transport system of the halophilic purple nonsulfur bacterium Rhodospirillum salinarum. 1. A functional and thermodynamic analysis of the respiratory chain in aerobically and photosynthetically grown cells

Plasma membranes isolated from cells of the halophilic purple nonsulfur bacterium Rhodospirillum salinarum grown in light or in the dark were examined. Membranes isolated from cells grown aerobically in the dark contained three b-type and two c-type membrane-bound cytochromes with E _m,7 of +180, +72 and –5 mV (561–575 nm), and +244 and +27 mV (551–540 nm), respectively. Conversely, membranes isolated from cells grown anaerobically in the light contained two b-type and five c-type haems with E _m,7 of +60 and –45 mV and +290, +250, +135, –20 and –105 mV, respectively. In addition to haems of the b- and c-type, two haems of the a-type (E _m,7 of +325 and +175 mV) were present only in cells grown in the dark. Four soluble cytochromes of the c type, but not cytochrome c ₂, along with two high-potential iron-sulfur proteins (HiPIP iso-1 and iso-2) were also identified in cells grown aerobically. Inhibitory studies showed that 85–90% of the respiratory activity was blocked by very low concentrations of cyanide, antimycin A and myxothiazol (50, 0.1 and 0.2 mM, respectively). These results taken together were interpreted to show that the oxidative electron transport chain of Rsp. salinarum is linear, leading to a membrane-bound oxidase of the aa ₃ type in cells grown in the dark, while no significant cytochrome oxidase activity is catalyzed by photosynthetic membranes. These features suggest that this halophilic species is unique among the genus Rhodospirillum and that it also differs from other facultative phototrophs (e.g., Rhodobacter species) in that it does not contain either cytochrome c ₂ or a branched respiratory chain. Received: 25 February 1997 / Accepted: 20 May 1997     

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 membrane bound c-type cytochromes (cyt) involved in photosynthetic electron transfer of the halophilic phototrophic bacterium Rhodospirillum (Rs.) salinarum 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, band at 556 nm) was able to reduce the photo-oxidized reaction center (P⁺) in a sub-millisecond ( 20 s) time scale reaction, acting most likely as the direct electron donor to P⁺). The midpoint potential of the primary electron donor (Em_7.0 = + 455 mV) was found to be close to that reported for the primary donor of the non-halophilic Rhodospirillum species Rs. rubrum, whereas the quinone primary electron acceptor (Qa) was different showing 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 of 40 kDa was also isolated and referred to an orthodox cytochrome c₁). The present data on the photosynthetic apparatus, along with the previous results on the respiratory system [Moschettini et al. (1997) Arch Microbiol 168: 302-309], suggest that Rs. salinarum is biochemically distinct from Rs. rubrum, the most representative specie of the genus.