Nitrous oxide reduction by two partial denitrifying bacteria requires denitrification intermediates that cannot be respired.

Denitrification is a form of anaerobic respiration wherein nitrate (NO3 -) is sequentially reduced via nitrite (NO2 -), nitric oxide, and nitrous oxide (N2O) to dinitrogen gas (N2) by four reductase enzymes. Partial denitrifying bacteria possess only one or some of these four reductases and use them as independent respiratory modules. However, it is unclear if partial denitrifiers sense and respond to denitrification intermediates outside of their reductase repertoire. Here, we tested the denitrifying capabilities of two purple nonsulfur bacteria, Rhodopseudomonas palustris CGA0092 and Rhodobacter capsulatus SB1003. Each had denitrifying capabilities that matched their genome annotation; CGA0092 reduced NO2 - to N2, and SB1003 reduced N2O to N2. For each bacterium, N2O reduction could be used both for electron balance during growth on electron-rich organic compounds in light and for energy transformation via respiration in darkness. However, N2O reduction required supplementation with a denitri fication intermediate, including those for which there was no associated denitrification enzyme. For CGA0092, NO3 - served as a stable, non-catalyzable molecule that was sufficient to activate N2O reduction. Using a β-galactosidase reporter, we found that NO3 - acted, at least in part, by stimulating N2O reductase gene expression. In SB1003, NO2 - but not NO3 - activated N2O reduction, but NO2 - was slowly removed, likely by a promiscuous enzyme activity. Our findings reveal that partial denitrifiers can still be subject to regulation by denitrification intermediates that they cannot use. [ABSTRACT FROM AUTHOR]