MUTANTS OF PSEUDOMONAS-FLUORESCENS DEFICIENT IN DISSIMILATORY NITRITE REDUCTION ARE ALSO ALTERED IN NITRIC-OXIDE REDUCTION

Five Tn5 mutants of Pseudomonas fluorescens AK-15 deficient in dissimilatory reduction of nitrite were isolated and characterized. Two insertions occurred inside the nitrite reductase structural gene (nirS) and resulted in no detectable nitrite reductase protein on a Western immunoblot. One mutant had Tn5 inserted inside nirC, the third gene in the same operon, and produced a defective nitrite reductase protein. Two other mutants had insertions outside of this nir operon and also produced defective proteins. All of the Nir- mutants characterized showed not only loss of nitrite reductase activity but also a significant decrease in nitric oxide reductase activity. When cells were incubated with 15NO in H-2 O-18, about 25% of the oxygen found in nitrous oxide exchanged with H2O. The extent of exchange remained constant throughout the reaction, indicating the incorporation of O-18 from H-2 O-18 reached equilibrium rapidly. In all nitrite reduction-deficient mutants, less than 4% of the O-18 exchange was found, suggesting that the hydration and dehydration step was altered. These results indicate that the factors involved in dissimilatory reduction of nitrite influenced the subsequent NO reduction in this organism.