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Nitrosomonas europaea has two copies of the operon encoding ammonia monooxygenase (AMO). The nucleotide sequences of the two copies ofamoA were obtained, and they were found to differ by one nucleotide. To determine if both copies ofamoA were functional, insertional mutagenesis was performed to inactivate either copy ofamoA alone. A DNA cassette containing thelacZ andkan genes inserted intoamoA was constructed. Mutagenesis was done by using transformation and homologous recombination to mobilize the cassette into the chromosomal copies ofamoA . Mutations were obtained in both copies ofamoA . Either copy ofamoA was sufficient to support growth when the other copy was disrupted. However, inactivation of one copy ofamoA , but not the other, resulted in slower growth. Measurements of ammonia-dependent O2 consumption, which depends on AMO, confirmed that the slower-growing mutant had lower activity while the faster-growing mutant had near wild-type levels of activity. Similarly, as measured by [14 C]acetylene label incorporation, there was less active AMO present in the slower-growing mutant than in the faster-growing mutant or in the wild type. Northern blot analysis of transcription likewise showed that the slower-growing mutant had less full-sized AMO mRNA.

Mutagenesis and Expression of amo , Which Codes for Ammonia Monooxygenase in Nitrosomonas europaea