Tellurite processing by cells of Rhodobacter capsulatus involves a periplasmic step where the oxyanion causes a malfunction of the cytochrome c maturation system

Tellurite (TeO32−) is an oxyanion of increasing concern for soils and water bodies as tellurite is toxic to prokaryotes and eukaryotes. In cells of Rhodobacter capsulatus, a facultative anoxygenic photosynthetic bacterium which tolerates up to 1 mM tellurite, the assembly of c-type cytochromes (cyts) occurs in the periplasm and involves several proteins ensuring the correct redox-state of the cyt cmaturation process (Ccm). Here it is shown that a sub-lethal tellurite concentration (0.2 mM) reduces the amount of cyts c during the early growth-phase of R. capsulatus. This effect is due to a malfunction of the Ccm system and not to an effect on the transcriptional expression of the cyt c genes. Further, growth of mutants MT-G4/S4 and MD21, with mutations at the level of cyt c2 (ΔcycA) and of the Ccm system (ΔdsbA/ΔccdA), respectively, was more affected by tellurite than wild type cells. Both MT-G4/S4 and MD21 mutants, featured by a cyt c2 deficiency, showed a significant decrease in their capacity to reduce TeO32− into Te0, thereby implying the need of a full amount of periplasmic cyt c2 in the microbial-driven speciation of the metalloid. These findings show, for the first time, that tellurite processing by cells starts at the periplasmic level.