REDUCTION AND REMOVAL OF HEPTAVALENT TECHNETIUM FROM SOLUTION BY ESCHERICHIA-COLI

Anaerobic, but not aerobic, cultures of Escherichia coli accumulated T c(VII) and reduced it to a black insoluble precipitate, Tc was the pre dominant element detected when the precipitate was analyzed by proton- induced X-ray emission, Electron microscopy in combination with energy -dispersive X-ray analysis showed that the site of Tc deposition was i ntracellular, It is proposed that Te precipitation was a result of enz ymatically mediated reduction of Tc(VII) to an insoluble oxide, Format e was an effective electron donor for Tc(VII) reduction which could be replaced by pyruvate, glucose, or glycerol but not by acetate, lactat e, succinate, or ethanol, Mutants defective in the synthesis of the tr anscription factor FNR, in molybdenum cofactor (molybdopterin guanine dinucleotide [MGD]) synthesis, or in formate dehydrogenase H synthesis were all defective in Tc(VII) reduction, implicating a role for the f ormate hydrogenlyase complex in Tc(VII) reduction, The following obser vations confirmed that the hydrogenase III (Hyc) component of formate hydrogenlyase is both essential and sufficient for Tc(VII) reduction: (i) dihydrogen could replace formate as an effective electron donor fo r Tc(VII) reduction by wild-type bacteria and mutants defective in MGD synthesis; (ii) the inability of fnr mutants to reduce Tc(VII) can be suppressed phenotypically by growth with 250 mu M Ni2+ and formate; ( iii) Tc(VII) reduction is defective in a hyc mutant; (iv) the ability to reduce Tc(VII) was repressed during anaerobic growth in the presenc e of nitrate, but this repression was counteracted by the addition of formate to the growth medium; (v) H-2, but not formate, was an effecti ve electron donor for a Sel(-) mutant which is unable to incorporate s elenocysteine into any of the three known formate dehydrogenases of E. coli. This appears to be the first report of Hyc functioning as an H- 2-oxidizing hydrogenase or as a dissimilatory metal ion reductase in e nteric bacteria.