The Thiol:Disulfide Oxidoreductase DsbB Mediates the Oxidizing Effects of the Toxic Metalloid Tellurite (TeO 3 2− ) on the Plasma Membrane Redox System of the Facultative Phototroph Rhodobacter capsulatus

Abstract

The highly toxic oxyanion tellurite (TeO ₃ ²⁻ ) is a well known pro-oxidant in mammalian and bacterial cells. This work examines the effects of tellurite on the redox state of the electron transport chain of the facultative phototroph Rhodobacter capsulatus , in relation to the role of the thiol:disulfide oxidoreductase DsbB. Under steady-state respiration, the addition of tellurite (2.5 mM) to membrane fragments generated an extrareduction of the cytochrome pool ( c - and b -type hemes); further, in plasma membranes exposed to tellurite (0.25 to 2.5 mM) and subjected to a series of flashes of light, the rate of the QH ₂ :cytochrome c (Cyt c ) oxidoreductase activity was enhanced. The effect of tellurite was blocked by the antibiotics antimycin A and/or myxothiazol, specific inhibitors of the QH ₂ :Cyt c oxidoreductase, and, most interestingly, the membrane-associated thiol:disulfide oxidoreductase DsbB was required to mediate the redox unbalance produced by the oxyanion. Indeed, this phenomenon was absent from R. capsulatus MD22, a DsbB-deficient mutant, whereas the tellurite effect was present in membranes from MD22/pDsbB ^WT , in which the mutant gene was complemented to regain the wild-type DsbB phenotype. These findings were taken as evidence that the membrane-bound thiol:disulfide oxidoreductase DsbB acts as an “electron conduit” between the hydrophilic metalloid and the lipid-embedded Q pool, so that in habitats contaminated with subinhibitory amounts of Te ^IV , the metalloid is likely to function as a disposal for the excess reducing power at the Q-pool level of facultative phototrophic bacteria.