Oxygen-dependent control of respiratory nitrate reduction in mycelium of Streptomyces coelicolor A3(2).

Several members of the obligately aerobic genus Streptomyces are able to reduce nitrate, catalyzed by Nar-type respiratory nitrate reductases. A unique feature of Streptomyces coelicolor A3(2) compared with other streptomycetes is that it synthesizes three nonredundant Nar enzymes. In this study, we show that Nar2 is the main Nar enzyme active in mycelium and could characterize the conditions governing its synthesis. Nar2 was present at low levels in aerobically cultivated mycelium, but synthesis was induced when cultures were grown under oxygen limitation. Growth in the presence of high oxygen concentrations prevented the induction of Nar2 synthesis. Equally, an abrupt shift from aerobiosis to anaerobiosis did not result in the immediate induction of Nar2 synthesis. This suggests that the synthesis of Nar2 is induced during a hypoxic downshift, probably to allow maintenance of a proton gradient during the transition to anaerobiosis. Although no Nar2 could be detected in freshly harvested mature spores, synthesis of the enzyme could be induced after long-term (several days) incubation of these resting spores under anaerobic conditions. Induction of Nar2 synthesis in spores was linked to transcriptional control. Nar2 activity in whole mycelium was strictly dependent on the presence of a putative nitrate transporter, NarK2. The oxygen-dependent inhibition of nitrate reduction by Nar2 was mediated by NarK2-dependent nitrate:nitrite antiport. This antiport mechanism likely prevents the accumulation of toxic nitrite in the cytoplasm. A deletion of the narK2 gene had no effect on Nar1-dependent nitrate reduction in resting spores. Together, our results indicate redox-dependent transcriptional and posttranslational control of nitrate reduction by Nar2.