Codenitrification and denitrification are dual metabolic pathways through which dinitrogen evolves from nitrate in Streptomyces antibioticus

We screened actinomycete strains for dinitrogen (N-2)-producing activity and discovered that Streptomyces antibioticus B-546 evolves N-2 and some nitrous oxide (N2O) from nitrate (NO3-). Most of the N2 that evolved from the heavy isotope ([N-15]NO3-) was (NN)-N-15-N-14, indicating that this nitrogen species consists of two atoms, one arising from NO3- and the other from different sources. This phenomenon is similar to codenitrification in fungi. The strain also evolved less, but significant, amounts of (NN)-N-15-N-15 from [N-15]NO3- in addition to (NNO)-N-15-N-15 with concomitant cell growth. Prior to the production of N-2 and N2O, NO3- was rapidly reduced to nitrite (NO2-) accompanied by distinct cell growth, showing that the actinomycete strain is a facultative anaerobe that depends on denitrification and nitrate respiration for anoxic growth. The cell-free activities of denitrifying enzymes could be reconstituted, supporting the notion that the (NN)-N-15-N-15 and (NNO)-N-15-N-15 species are produced by denitrification from NO3- via NO2-. We therefore demonstrated a unique system in an actinomycete that produces gaseous nitrogen (N-2 and N2O) through both denitrification and codenitrification. The predominance of codenitrification over denitrification along with oxygen tolerance is the key feature of nitrate metabolism in this actinomycete.