Abstract
Bacterial denitrification is a respiratory process that is a major source and sink of the potent greenhouse gas nitrous oxide. Many denitrifying bacteria can adjust to life in both oxic and anoxic environments through differential expression of their respiromes in response to environmental signals such as oxygen, nitrate and nitric oxide. We used steady-state oxic and anoxic chemostat cultures to demonstrate that the switch from aerobic to anaerobic metabolism is brought about by changes in the levels of expression of relatively few genes, but this is sufficient to adjust the configuration of the respirome to allow the organism to efficiently respire nitrate without the significant release of intermediates, such as nitrous oxide. The regulation of the denitrification respirome in strains deficient in the transcription factors FnrP, Nnr and NarR was explored and revealed that these have both inducer and repressor activities, possibly due to competitive binding at similar DNA binding sites. This may contribute to the fine tuning of expression of the denitrification respirome and so adds to the understanding of the regulation of nitrous oxide emission by denitrifying bacteria in response to different environmental signals.
Highlights
Continuous cultures were established in which P. denitrificans was allowed to grow to a steady-state biomass under aerobic conditions
In the aerobic continuous culture nitrate levels remained constant at 20 mM throughout the experimental period (Fig. 2B). This indicated that neither denitrification nor assimilatory NO23 reduction pathways were operational and that the steady-state culture was sustained only by aerobic respiration
After the period of transition associated with the shift from aerobic to anaerobic metabolism the biomass in the steady-state was only slightly less than in the WT cultures (Table 1), the NO23 levels in the bioreactor continued to decrease over the lifetime of the experiment with 10 mM NO23 remaining in the vessel after 120 h (Fig. 2F)
Summary
Denitrifying bacteria play an important role in determining the fate of reactive nitrogen in both terrestrial and aquatic environments, especially when oxygen (O2) is limiting (Thomson et al, 2012). The regulation of the denitrification respirome in strains deficient in the transcription factors FnrP, Nnr and NarR was explored and revealed that these have both inducer and repressor activities, possibly due to competitive binding at similar DNA binding sites This may contribute to the fine tuning of expression of the denitrification respirome and so adds to the understanding of the regulation of nitrous oxide emission by denitrifying bacteria in response to different environmental signals. The enzymes catalysing these reactions serve as termini in a branched respiratory network. Together these data provide new insight into how the respirome is remodelled in response to the transition between aerobic and anaerobic conditions
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