Abstract
Intracellular ATP levels in whole cells of Methanococcus voltae respond to electron transfer coupled to methanogenesis. ATP synthesis can also be induced by an artificially imposed transmembrane electrical potential [formed by electrogenic movement outwards of potassium (induced by valinomycin) or of protons (induced by an uncoupler], or by a pH gradient (acid outside). These results implicate the existence of a reversible ATPase coupled to electrogenic movement of an ion(s) other than potassium or proton, and that ionophores are competent to catalyze ion movement across the cytoplasmic membrane of this organism (which is the sole membrane structure in this species). ATP synthesis driven by electron transfer is, however, insensitive to the addition of such ionophores. These results indicate that although cells possess an ion-translocating ATPase (possibly involved in the maintenance of internal ionic composition homeostasis), methanogenesis-driven ATP synthesis does not involve the intermediacy of a transmembrane ion gradient. Primarily because methane formation has been previously demonstrated to involve true electron transfer, substrate-level phosphorylation (at least in analogy to other systems) has been generally ruled out. The results presented here suggest that at least one methanogenic bacterium may use a direct linkage of ATP synthesis to electron transfer.