THE REDUCTION OF NITROUS-OXIDE TO DINITROGEN BY ESCHERICHIA-COLI

Escherichia coli K12 reduces nitrous oxide stoichiometrically to molec ular nitrogen with rates of 1.9 mumol/h x mg protein. The activity is induced by anaerobiosis and nitrate. N2-formation from N2O is inhibite d by C2H2 (K(i) approximately 0.03 mM in the medium) and nitrite (K(i) = 0.3 mM) but not by azide. A mutant defective in FNR synthesis is un able to reduce N2O to N2. The reaction in the wild type could routinel y be followed by gas chromatography and alternatively by mass spectrom etry measuring the formation of N-15(2) from (N2O)-N-15 The enzyme cat alyzing N2O-reduction in E. coli could not be identified; it is probab ly neither nitrate reductase nor nitrogenase. E. coli does not grow wi th N2O as sole respiratory electron acceptor. N2O-reduction might not have a physiological role in E. coli, and the enzyme involved might ca talyze something else in nature, as it has a low affinity for the subs trate N2O (apparent K(m) approximately 3.0 mM). The capability for N2O -reduction to N2 is not restricted to E. coli but is also demonstrable in Yersinia kristensenii and Buttiauxella agrestis of the Enterobacte riaceae. E. coli is able to produce NO and N2O from nitrite by nitrate reductase, depending on the assay conditions. In such experiments NO2 - is not reduced to N2 because of the high demand for N2O of N2O-reduc tion and the inhibitory effect of NO2- on this reaction.