Detoxification and reduction of selenite to elemental red selenium by Frankia

Four Frankia strains (EuI1c, CN3, ACN14a and CcI3) were tested for selenite tolerance. Frankia inefficax strain EuI1c was resistant to selenite with a MIC value of 518.8 µg ml^-1. After 48 h incubation with selenite, a reddish precipitate began to appear in these cultures. The red color suggests the reduction of the toxic, soluble, and colorless sodium selenite (Na₂SeO₃^2-) to the nontoxic, insoluble, and red colored elemental selenium (Seº). Analysis showed F. inefficax strain EuI1c cultures exposed to 17.3 and 86.5 µg ml^-1selenite completely reduced all of the selenite after 5 and 8 days, respectively. When observed under Scanning Electron Microscopy, selenite-resistant F. inefficax strain EuI1c grown with selenite formed nanosphere particles on the hyphal surface as free deposits or in aggregates and inside the hyphae. EDAX analysis of the nanosphere particles determined that they are composed of selenium with up to 27.3-fold increase in intensity as compared to control cells. FTIR Spectroscopy of selenite-stressed cells showed cell surface changes in fatty acids, polysaccharides, carbohydrates and phosphate groups. This result suggests a mechanism for selenite reduction and nanosphere transport through cell membrane in this strain. Native gel electrophoresis of extracted cell-free protein revealed one band showing activity after staining with selenite and NADH. SDS-PAGE analysis revealed the presence of several bands with one dominant band of 37.8 kDa. Mass spectrometry analysis of the bands determined that the main proteins were a periplasmic-binding protein, sulfate ABC transporter and extracellular ligand-binding receptor.