The Thiol:Disulfide Oxidoreductase DsbB Mediates the Oxidizing Effects of the Toxic Metalloid Tellurite (TeO₃²⁻) on the Plasma Membrane Redox System of the Facultative PhototrophRhodobacter 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.In the environment, tellurium (Te) exists in its elementalTe 0 -inorganic-telluride (Te 2Ϫ ), tellurite (TeO 3 2Ϫ ), and tellurate (TeO 4 2Ϫ )-and organic-dimethyl telluride (CH 3 TeCH 3 )-forms (8). Of these, the toxic oxyanion forms, TeO 3 2Ϫ and TeO 4 2Ϫ , are more common than and are highly soluble compared to nontoxic elemental tellurium, Te 0 (38). Tellurium is widely used in the electronics industry, for photoreceptors, thermocouples, and batteries, but also in metallurgical processes and as an additive to industrial glasses (8). As a result, microorganisms are now becoming exposed to abnormal concentrations of this element, and bacterial species resistant to tellurium can easily be isolated from industrial sludge (38). However, research into the anthropogenic emission of Tebased compounds is scarce, and the implications for selection of microorganisms resistant to tellurite (TeO 3 2Ϫ ) and tellurate (TeO 4 2Ϫ ) are largely unexplored (40). Tellurite is more toxic to mammalian cells (43) and microorganisms (38) than are several heavy metals, e.g., mercury, cadmium, zinc, chromium, and cobalt, which are objects of major public health concern (38). Depending on the strain, the concentration of tellurite inhibiting microbial growth ranges from 1 to 1,000 g/ml (34,38,(46)(47)(48). Microorganisms counteract tellurite (TeO 3 2Ϫ ) toxicity in several ways, namely, by (i) decreasing its uptake, (ii) enhancing its effl...

Pseudomonas fluorescens' view of the periodic table

“…It has been observed for some time that in the environment, many Pseudomonas spp. accumulate inactivating mutations specifically in a twocomponent regulatory system comprised of the GacS and GacA proteins (van den Broek et al, 2005). Recently it was shown that gacSmutants of P. aeruginosa biofilms respond to metal and other stresses by producing hyper-biofilm forming, stress-resistant phenotypic variants (Davies et al, 2007), and we have observed a similar effect in P. fluorescens (our unpublished data).…”

“…In terms of toxicity this has implications in numerous cellular processes involving redox reactions. As an example, recent work with membranes of Rhodobacter capsulatus and tellurite suggests that tellurite may be acting as an electron sink, pulling excess reducing power from the electron transport chain (Borsetti et al ., 2007). The importance of redox chemistry in toxicity is also exemplified by the ability of redox‐active metals, such as Fe, Cu, Ni, Cr, V and Zn, to cause oxidative stress (Stohs and Bagchi, 1995).…”

The putative phosphate transporter PitB (PP1373) is involved in tellurite uptake in Pseudomonas putida KT2440

“…Tellurite is known to activate the machinery of resistance to oxidative stress in E. coli and Pseudomonas pseudoalcaligenes that includes NADPH/glutathione-mediated reactions [2,4,13,15]. In Rhodobacter capsulatus, a thiol:disulfide oxidoreductase was shown to participate in electron transport resulting in the reduction of tellurite [16]. Several tellurium-resistant genes have been identified in E. coli (ter, kilA and tehAB operons) [5] and other bacteria such as R. sphaeroides ( One aspect not defined for all bacterial systems regarding the tellurite metabolism is its transport system.…”

Microbial-Based Bioremediation of Selenium and Tellurium Compounds

“…To date, the proposed mechanisms of Te-oxyanions bioconversion in microorganisms are similar to those described for Se-species [13,56,88,104,108]. Further, TeO 3 2− processing in microorganisms have been ascribed to enzymatic reductions by periplasmic or cytoplasmic oxidoreductases [107,109], such as nitrate reductases [109,110], catalases [111] and thiol:disulfide oxidoreductase [112]. However, the function of all these enzymes for bioconverting Te-oxyanions appears to be not specific, leading to a low resistance level toward Te-species in these microorganisms.…”