Abstract

Acidithiobacillus ferrooxidans is an acidophilic chemolithoautotrophic bacterium that can grow in the presence of either the weak reductant Fe 2+, or reducing sulfur compounds that provide more energy for growth than Fe2+. We have previously shown that the uphill electron transfer pathway between Fe2+ and NAD+ involved a bc1 complex that functions only in the reverse direction [J. Bacteriol. 182, (2000) 3602]. In the present work, we demonstrate both the existence of a bc 1 complex functioning in the forward direction, expressed when the cells are grown on sulfur, and the presence of two terminal oxidases, a bd and a ba3 type oxidase expressed more in sulfur than in iron-grown cells, besides the cytochrome aa3 that was found to be expressed only in iron-grown cells. Sulfur-grown cells exhibit a branching point for electron flow at the level of the quinol pool leading on the one hand to a bd type oxidase, and on the other hand to a bc1?ba3 pathway. We have also demonstrated the presence in the genome of transcriptionally active genes potentially encoding the subunits of a bo3 type oxidase. A scheme for the electron transfer chains has been established that shows the existence of multiple respiratory routes to a single electron acceptor O2. Possible reasons for these apparently redundant pathways are discussed. © 2004 Elsevier B.V. All rights reserved.