The transcription level of amoA mRNA encoding a subunit of ammonia monooxygenase (AMO) in ammonia-oxidizing bacteria (AOB) was quantified by reverse transcription-polymerase chain reaction (RT-PCR) methods in combination with real-time PCR technology. The effects of ammonia concentration and dissolved oxygen (DO) on the transcription levels of amoA mRNA and 16S rRNA in AOB were evaluated in batch experiments with nitrifying sludge taken from a lab-scale reactor treating artificial wastewater. A batch incubation without ammonia resulted in a rapid decrease, within four hours, in the transcription level of amoA mRNA to as low as 1/10 of that at the beginning of the experiment, while the 16S rRNA level in AOB was almost constant. After subsequent incubation with 3 mM ammonia for eight hours, a small increase in the transcription level of amoA mRNA occurred, but ammonia oxidation proceeded in the interim. Copy numbers of amoA mRNA showed an almost fixed value for over eight hours in the absence of dissolved oxygen.

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