Membrane-Associated Hexavalent Chromium Reductase of Bacillus megaterium TKW3 with Induced Expression

Abstract

Hexavalent chromium (Cr 6+ ) is a highly harmful pollutant, which can be detoxified and precipitated through reduction to Cr 3+ . Bacillus megaterium TKW3 previously isolated from chromium-contaminated marine sediments was capable of reducing Cr 6+ in concomitance with metalloids (Se 4+ , Se 6+ , and As 5+ ). Notwithstanding approximately 50% inhibition, it was the first report of simultaneous bacterial reduction of Cr 6+ and Se 4+ (to elemental Se). No significant difference was observed among electron donors (glucose, maltose, and mannitol) on Cr 6+ reduction by B. megaterium TKW3. The reduction was constitutive and determined to be non-plasmid mediated. Peptide mass fingerprints (PMF) revealed a novel aerobic membrane-associated reductase with Cr6'-induced expression and specific reductive activity (in nmol Cr 6+ /mg protein/min) of 0.220 as compared with 0.087 of the soluble protein fraction. Respiratory inhibitor NaN 3 did not interfere with the reductase activity. Transmission electron microscopy with energy dispersive X-ray (TEM-EDX) analysis confirmed the aggregation of reduced chromium along the intracellular membrane region. Future identification of the N-terminal amino acid sequence of this reductase will facilitate purification and understanding of its enzymatic action.