In Nitrosomonas europaea, ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO) catalyse the oxidation of ammonia (NH3) to nitrite (NO2-). A transcript of 3500 bases hybridizes to probes for amoA and amoB (genes that code for AMO proteins). A transcript of 2100 bases hybridizes to probes for hao (the gene that codes for HAO). Induction of the mRNAs detected by amo and hao probes required the presence of ammonium (NH4+). To correlate new levels of mRNA with de novo activity, existent mRNA pools and AMO activity were depleted prior to induction by NH4+. The mRNAs of AMO and HAO were depleted by depriving the cells of energy for at least 8 h; AMO activity was inactivated with acetylene (C2H2) after mRNA depletion. In cells treated this way, levels of new AMO mRNA and de novo AMO enzyme activity were correlated with increased NH4+ concentrations up to 1 mM after 3 h of incubation. HAO mRNA also increased in the NH4(+)-treated cells. Other proteins and RNAs induced by NH4+ were detected in 14CO2-labelling experiments. The AMO and HAO mRNAs were preferentially synthesized during energy-limiting conditions.