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 bc1 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 (Em,7 of +244 +/- 8, +24 +/- 3, -163 +/- 11 mV), four c-type cytochromes (Em,7 of +280 +/- 10, +210 +/- 5, +125 +/- 8, and 20 +/- 3 mV) and two a-type cytochromes (Em,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 microM) and azide (I50 = 1 mM), while a soluble cytochrome c (c551, Em,7 = +217 +/- 2 mV) was shown to be the physiological electron carrier connecting the bc1 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 (Eh > or = +80 < or = +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.

The Complete Genome Sequence of Roseobacter denitrificans Reveals a Mixotrophic Rather than Photosynthetic Metabolism

“…A single colony of Roseobacter denitrificans strain OCh 114 was grown heterotrophically as described previously (8), and total DNA was isolated using proteinase K treatment followed by phenol extraction. The DNA was fragmented by kinetic shearing, and three shotgun libraries were generated: small-and mediuminsert libraries in pOTWI3 (using size fractions of 2 to 3 kb and 6 to 8 kb, respectively) and a large-insert fosmid library in pEpiFOS-5 (insert sizes ranging from 28 to 47 kb), which was used as a scaffold.…”

Oxygen intervention in the regulation of gene expression: the photosynthetic bacterial paradigm

“…Intriguingly, Prr homologues have also been identified in a member of this group of bacteria [90], and similarity of function was demonstrated by complementation studies in R. capsulatus mutant strains [90]. Perhaps, as alluded to by Candela et al [123], the oxygen requirement of these anoxygenic photosynthetic organisms is due to the absence of a suitable outlet for photosynthetically generated reducing power under anaerobic conditions. This hypothesis predicts that introducing genes into these organisms that would confer the ability to carry out carbon dioxide fixation as a suitable electron sink would then allow them to grow in the absence of oxygen.…”

“…Apparently photosynthesis activity is not obligatory in these organisms, but provides additional energy, perhaps offsetting respiratory rate changes that could occur as temperatures fluctuate, as an example [121]. These organisms are not capable of fixing carbon dioxide, and therefore the role of photosynthesis appears to be strictly energetic in an ancillary sense [121,123]. Clear evidence of this specialized role of photosynthesis in these organisms stems from an analysis of Bchl production, which reveals that light abolishes Bchl formation, and so photosynthesis is only as long-lived as is the limited availability of Bchl produced before illumination; Bchl levels will CMLS, Cell.…”

Influence of Light on Carbon Utilization in Aerobic Anoxygenic Phototrophs

“…The photosynthetic competence of the AAPs has been demonstrated by spectroscopic analyses (9,27,32) and infrared (IR) kinetic fluorescence measurements (16,21). It was shown that light exposure of AAP cells inhibits respiration (10,16) and increases their cellular ATP concentration (2,25). This indicates that AAPs are able to partially replace oxidative phosphorylation with photophosphorylation.…”