THE NASFEDCBA OPERON FOR NITRATE AND NITRITE ASSIMILATION IN KLEBSIELLA-PNEUMONIAE M5AL

Klebsiella pneumoniae can use nitrate and nitrite as sole nitrogen sou rces through the nitrate assimilation pathway. We previously identifie d structural genes for assimilatory nitrate and nitrite reductases, na sA and nasB, respectively. We report here our further identification o f four genes, nasFEDC, upstream of the nasBA genes. The nasFEDCBA gene s probably form an operon. Mutational and complementation analyses ind icated a that both the nasC and nasA genes are required for nitrate as similation. The predicted NASC protein is homologous to a variety of N ADH-dependent oxidoreductases. Thus, the NASC protein probably mediate s electron transfer from NADH to the NASA protein, which contains the active site for nitrate reduction. The deduced NASF, NASE, and NASD pr oteins are homologous to the NRTA, NRTB, and NRTD proteins, respective ly, that are involved in nitrate uptake in Synechococcus sp. (T. Omata , X. Andriesse, and A. Hirano, Mel. Gen. Genet. 236:193-202, 1993). Mu tational and complementation studies indicated that the nasD gene is r equired for nitrate but not nitrite assimilation. By analogy,vith the Synechococcus nrt genes, we propose that the nasFED genes are involved in nitrate transport in K. pneumoniae.