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Two different biochemical pathways are known in nitrification by heterotrophic microorganisms. In contrast, the biochemical mechanism of heterotrophic nitrification and its regulatory system have remained poorly understood. In addition, nitrogen removal by a combination of heterotrophic nitrification and aerobic denitrification is a promising technology for efficient wastewater treatment and has been actively studied 6– 8. The biological activity of heterotrophic nitrification has been reported mainly in soil bacteria of the phylum Proteobacteria, but some species of actinomycetes and ascomycetes have been also known to exhibit nitrifying activity 4, 5. However, due to their large biomass and species richness in soil, overall nitrifying activity by heterotrophic bacteria cannot be ignored, especially in acidic soils such as coniferous forests where the nitrification by autotrophic microorganisms is inhibited 1– 3. Generally, the nitrifying activity per cell of heterotrophic bacteria is very low compared to that of the autotrophic nitrifying microorganisms. Some heterotrophic bacteria possess the biological ability to oxidize ammonia and to form nitrite or nitrate. Our results provide valuable insight into the biochemical mechanism of heterotrophic nitrification. We observed a significant increase in the expression levels of four gene clusters: pod cluster containing the gene encoding pyruvic oxime dioxygenase (POD), podh cluster containing the gene encoding a POD homolog (PODh), suf cluster involved in an iron-sulfur cluster biogenesis, and dnf cluster involved in a novel hydroxylamine oxidation pathway in the nitrifying cells.
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Expression levels of genes involved in primary metabolism did not change significantly in the cells at the exponential growth phase under both conditions.
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faecalis cultivated in media whose C/N ratios were 5 and 20, respectively, to evaluate the fluctuations of gene expression during induction of heterotrophic nitrification. Comparative transcriptome analysis was performed using nitrifying and non-nitrifying cells of A. faecalis was cultivated in a medium containing pyruvate and ammonia as the sole carbon and nitrogen sources, respectively, high concentrations of nitrite accumulated in the medium whose carbon/nitrogen (C/N) ratio was lower than 10 during the exponential growth phase, while the accumulation was not observed in the medium whose C/N ratio was higher than 15. Alcaligenes faecalis is a heterotrophic nitrifying bacterium that oxidizes ammonia and generates nitrite and nitrate.