Effect of ammonia starvation on hydroxylamine oxidoreductase activity of Nitrosomonas europaea

A technique for detection of the activity of hydroxylamine oxidoreductase (HAO) involving denaturing SDS-polyacrylamide gels was developed. The activity of HAO of Nitrosomonas europaea was assayed using this technique, which revealed a single active band of 140 kDa. The HAO activity of other ammonia-oxidizers was also resistant to SDS, the molecular weights being identical to that of N. europaea. N. europaea cells starved of ammonia for up to 72 h retained a considerable amount of HAO, as detected on Western blot analysis, and a significant level of its activity, as found on assaying at the end of the starvation period. Only after 4 h incubation of starved N. europaea cells with 2.0 mM ammonia was some increase in the HAO level observed. The results indicate that HAO remains highly stable during ammonia starvation of N. europaea.