Energy conservation involving 2 respiratory circuits

Abstract

Chemiosmosis and substrate-level phosphorylation are the 2 mechanisms employed to form the biological energy currency adenosine triphosphate (ATP). During chemiosmosis, a transmembrane electrochemical ion gradient is harnessed by a rotary ATP synthase to phosphorylate adenosine diphosphate to ATP. In microorganisms, this ion gradient is usually composed ofH+, but it can also be composed of Na+. Here, we show that the strictly anaerobic rumen bacteriumPseudobutyrivibrio ruminispossesses 2 ATP synthases and 2 distinct respiratory enzymes, the ferredoxin:NAD+oxidoreductase (Rnf complex) and the energy-converting hydrogenase (Ech complex). In silico analyses revealed that 1 ATP synthase isH+-dependent and the other Na+-dependent, which was validated by biochemical analyses. Rnf and Ech activity was also biochemically identified and investigated in membranes ofP. ruminis. Furthermore, the physiology of the rumen bacterium and the role of the energy-conserving systems was investigated in dependence of 2 different catabolic pathways (the Embden–Meyerhof–Parnas or the pentose–phosphate pathway) and in dependence of Na+availability. Growth ofP. ruminiswas greatly stimulated by Na+, and a combination of physiological, biochemical, and transcriptional analyses revealed the role of the energy conserving systems inP. ruminisunder different metabolic scenarios. These data demonstrate the use of a 2-component ion circuit forH+bioenergetics and a 2nd 2-component ion circuit for Na+bioenergetics in a strictly anaerobic rumen bacterium. In silico analyses infer that these 2 circuits are prevalent in a number of other strictly anaerobic microorganisms.