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 of [Formula: see text], but it can also be composed of Na⁺ Here, we show that the strictly anaerobic rumen bacterium <i>Pseudobutyrivibrio ruminis</i> possesses 2 ATP synthases and 2 distinct respiratory enzymes, the ferredoxin:[Formula: see text] oxidoreductase (Rnf complex) and the energy-converting hydrogenase (Ech complex). In silico analyses revealed that 1 ATP synthase is [Formula: see text]-dependent and the other Na⁺-dependent, which was validated by biochemical analyses. Rnf and Ech activity was also biochemically identified and investigated in membranes of <i>P. ruminis</i> 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 of <i>P. ruminis</i> was greatly stimulated by Na⁺, and a combination of physiological, biochemical, and transcriptional analyses revealed the role of the energy conserving systems in <i>P. ruminis</i> under different metabolic scenarios. These data demonstrate the use of a 2-component ion circuit for [Formula: see text] 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.