Abstract
Shewanella species are facultative anaerobic bacteria that colonize redox-stratified habitats where O2 and nutrient concentrations fluctuate. The model species Shewanella oneidensis MR-1 possesses genes coding for three terminal oxidases that can perform O2 respiration: a bd-type quinol oxidase and cytochrome c oxidases of the cbb3-type and the A-type. Whereas the bd- and cbb3-type oxidases are routinely detected, evidence for the expression of the A-type enzyme has so far been lacking. Here, we investigated the effect of nutrient starvation on the expression of these terminal oxidases under different O2 tensions. Our results reveal that the bd-type oxidase plays a significant role under nutrient starvation in aerobic conditions. The expression of the cbb3-type oxidase is also modulated by the nutrient composition of the medium and increases especially under iron-deficiency in exponentially growing cells. Most importantly, under conditions of carbon depletion, high O2 and stationary-growth, we report for the first time the expression of the A-type oxidase in S. oneidensis, indicating that this terminal oxidase is not functionally lost. The physiological role of the A-type oxidase in energy conservation and in the adaptation of S. oneidensis to redox-stratified environments is discussed.
Highlights
IntroductionA-type oxidases are more efficient at transducing energy than cbb3-type oxidases and are the major terminal oxidases expressed under aerobic conditions
With a lower H+/e− ratio of 0.510,11
In a previous study on the multiple respiratory systems of S. oneidensis MR-1, it was proposed that the cbb3-type and c oxidase (Cox) are expressed under low and high O2 tensions, respectively, whereas the bd-type oxidase would be required for growth under microaerobic conditions and/or stressful conditions[15]
Summary
A-type oxidases are more efficient at transducing energy than cbb3-type oxidases and are the major terminal oxidases expressed under aerobic conditions. The cbb3-type oxidase was the predominant oxidase under aerobic conditions while, unexpectedly, the Cox oxidase had no physiological significance under the tested conditions In view of these results it was proposed that in the course of evolution the Cox was functionally lost[17]. We surmised that Cox could be expressed under certain specific conditions To verify this notion, the expression patterns of the terminal oxidases in S. oneidensis were investigated under nutrient-starved conditions and under different dissolved O2 tensions. The physiological role of Cox in the energy conservation and adaptation of S. oneidensis to stratified environments will be evaluated in the discussion section
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