The highly toxic oxyanion tellurite (TeO32−) enters the phototrophic bacterium Rhodobacter capsulatus via an as yet uncharacterized monocarboxylate transport system

The facultative phototroph Rhodobacter capsulatus takes up the highly toxic oxyanion tellurite when grown under both photosynthetic and respiratory growth conditions. Previous works on Escherichia coli and R. capsulatus suggested that tellurite uptake occurred through a phosphate transporter. Here we present evidences indicating that tellurite enters R. capsulatus cells via a monocarboxylate transport system. Indeed, intracellular accumulation of tellurite was inhibited by the addition of monocarboxylates such as pyruvate, lactate and acetate, but not by dicarboxylates like malate or succinate. Acetate was the strongest tellurite uptake antagonist and this effect was concentration dependent, being already evident at 1 μM acetate. Conversely, tellurite at 100 μM was able to restrict the acetate entry into the cells. Both tellurite and acetate uptakes were energy dependent processes, since they were abolished by the protonophore FCCP and by the respiratory electron transport inhibitor KCN. Interestingly, cells grown on acetate, lactate or pyruvate showed a high level resistance to tellurite, whereas cells grown on malate or succinate proved to be very sensitive to the oxyanion. Taking these data together, we propose that: (a) tellurite enters R. capsulatus cells via an as yet uncharacterized monocarboxylate(s) transporter, (b) competition between acetate and tellurite results in a much higher level of tolerance against the oxyanion and (c) the toxic action of tellurite at the cytosolic level is significantly restricted by preventing tellurite uptake.