Regulation of nitrogen metabolism in the nitrate-ammonifying soil bacteriumBacillus viretiand evidence for its ability to grow using N2O as electron acceptor

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Bacillus viretiis a nitrate-ammonifying bacterium and a partial denitrifier, reducing NO3−, NO2−, NO and N2O with NarG, NrfA, CbaA and NosZ respectively. Growth is optimized through successive use of the electron acceptors O2 and NO3−, followed by NO2−, NO and N2O. Fermentation takes place simultaneously with anaerobic respiration. When grown in batch culture with 5 mM initial NO3−, transcription ofnrfAwas high and most NO3− was reduced to NH4+. With 20 mM initial NO3−,nrfAtranscription was lower and more than 50% of the nitrate was recovered as NO, N2O and N2. Analysis of gene transcription patterns and corresponding gas kinetics indicated that O2 and NO2− or NO are main controllers ofnrfA,nirB,cbaAandnosZtranscription. This was corroborated by analyses of putative binding regions for specific transcriptional regulators. Furthermore, we demonstrate that N2O reduction inB. viretisupports growth. The highnosZtranscription but low N2O production seen at 5 mM NO3− implies that this organism can use N2O reductase to scavenge N2O from other organisms in the soil, thus possibly acting as a net sink for N2O.

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