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 (N(2)O) from nitrate (NO(3)(−)). Most of the N(2) that evolved from the heavy isotope ([(15)N]NO(3)(−)) was (15)N(14)N, indicating that this nitrogen species consists of two atoms, one arising from NO(3)(−) and the other from different sources. This phenomenon is similar to codenitrification in fungi. The strain also evolved less, but significant, amounts of (15)N(15)N from [(15)N]NO(3)(−) in addition to (15)N(15)NO with concomitant cell growth. Prior to the production of N(2) and N(2)O, NO(3)(−) was rapidly reduced to nitrite (NO(2)(−)) 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 (15)N(15)N and (15)N(15)NO species are produced by denitrification from NO(3)(−) via NO(2)(−). We therefore demonstrated a unique system in an actinomycete that produces gaseous nitrogen (N(2) and N(2)O) 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.