Identification and functional analysis of a nitrate assimilation operon nasACKBDEF from Amycolatopsis mediterranei U32.

Nitrate assimilation has been well studied for Gram-negative bacteria but not so much in the Gram-positive actinomycetes up to date. In a rifamycin SV-producing actinomycete, Amycolatopsis mediterranei strain U32, nitrate not only can be used as a sole nitrogen source but also remarkably stimulates the antibiotic production along with regulating the related metabolic enzymes. A gene cluster of nasACKBDEF was cloned from a U32 genomic library by in situ hybridization screening with a heterogeneous nasB probe and confirmed later by whole genome sequence, corresponding to the protein coding genes of AMED_1121 to AMED_1127. These genes were co-transcribed as an operon, concomitantly repressed by ammonium while activated with supplement of either nitrate or nitrite. Genetic and biochemical analyses identified the essential nitrate/nitrite assimilation functions of the encoded proteins, orderly, the assimilatory nitrate reductase catalytic subunit (NasA), nitrate reductase electron transfer subunit (NasC), nitrate/nitrite transporter (NasK), assimilatory nitrite reductase large subunit (NasB) and small subunit (NasD), bifunctional uroporphyrinogen-III synthase (NasE), and an unknown function protein (NasF). Comparing rifamycin SV production and the level of transcription of nasB and rifE from U32 and its individual nas mutants in Bennet medium with or without nitrate indicated that nitrate assimilation function encoded by the nas operon played an essential role in the "nitrate stimulated" rifamycin production but had no effect upon the transcription regulation of the primary and secondary metabolic genes related to rifamycin biosynthesis.