Respiratory electron transport and light-induced energy transduction in membranes from the aerobic photosynthetic bacterium Roseobacter denitrificans

Abstract.

Membrane fragments isolated from the aerobic phototrophic bacterium Roseobacter denitrificans were examined. Ninety-five percent of the total NADH-dependent oxidative activity was inhibited either by antimycin A or myxothiazol, two specific inhibitors of the cytochrome bc ₁ complex, which indicates that the respiratory electron transport chain is linear. In agreement with this finding, light-induced oxygen uptake, an electron transport activity catalyzed by the "alternative quinol oxidase pathway" in membranes of several facultative phototrophic species, was barely detectable in membranes of Rsb. denitrificans. Redox titrations at 561–575 nm, 552–540 nm, and 602–630 nm indicated the presence of three b-type cytochromes (E_m,7 of +244±8, +24±3, –163±11 mV), four c-type cytochromes (E_m,7 of +280±10, +210±5, +125±8, and 20±3 mV) and two a-type cytochromes (E_m,7 of +335±15, +218±18 mV). The latter two a-type hemes were shown to be involved in cytochrome c oxidase activity, which was inhibited by both cyanide (I₅₀=2 µM) and azide (I₅₀=1 mM), while a soluble cytochrome c (c ₅₅₁, E_m,7=+217±2 mV) was shown to be the physiological electron carrier connecting the bc ₁ complex to the cytochrome c oxidase. A comparison of the ATP synthesis generated by continuous light in membranes of Rsb. denitrificans and Rhodobacter capsulatus showed that in both bacterial species photophosphorylation requires a membrane redox poise at the equilibrium (E_h≥+80≤+140 mV), close to the oxidation-reduction potential of the ubiquinone pool. These data, taken together, suggest that, although the photosynthetic apparatus of Rsb. denitrificans is functionally similar to that of typical anoxygenic phototrophs, e.g. Rba. capsulatus, the in vivo requirement of a suitable redox state at the ubiquinone pool level restricts the growth capacity of Rsb. denitrificans to oxic conditions.