Hydroxylamine metabolism in Pseudomonas PB16: involvement of a novel hydroxylamine oxidoreductase

Pseudomonas strain PB16, a Gram-negative heterotrophic nitrifying bacterium closely related to Pseudomonas azalaica on the basis of 16 S rDNA analysis, was able to use hydroxylamine as an additional energy source during growth in acetate limited chemostat cultures giving an increased biomass yield. In aerobically growing cells of Pseudomonas PB16 only 50% of supplemented hydroxylamine could be recovered as nitrite. In addition to nitrite, N2O could be detected in the chemostat off-gas, indicating combined heterotrophic nitrification and aerobic denitrification. The maximum specific hydroxylamine oxidizing activity observed was 450 nmol per min per mg dry weight, with a Ks of approximately 40 µm. Upon addition of hydroxylamine to the medium, Pseudomonas PB16 induced a soluble 132 KDa dimeric hydroxylamine oxidoreductase. The enzyme had a pH optimum of 9, and did not contain spectroscopic features typical for cytochromes, which is in contrast to hydroxylamine oxidoreductases fou nd in autotrophic bacteria.