CHARACTERIZATION OF THE NITRIC-OXIDE REDUCTASE-ENCODING REGION IN RHODOBACTER-SPHAEROIDES-2.4.3

A gene cluster which includes genes required for the expression of nit ric oxide reductase in Rhodobacter sphaeroides 2.4.3 has been isolated and characterized. Sequence analysis indicates that the two proximal genes in the cluster are the Nor structural genes. These two genes and four distal genes apparently constitute an operon. Mutational analysi s indicates that the two structural genes, norC and norB, and the gene s immediately downstream, norQ and norD, are required for expression o f an active Nor complex. The remaining two genes, nnrT and nnrU, are r equired for expression of both Nir and Nor. The products of norCBQD ha ve significant identity with products from other denitrifiers, whereas the predicted nnrT and nnrU gene products have no similarity with pro ducts corresponding to other sequences in the database. Mutational ana lysis and functional complementation studies indicate that the nnrT an d nnrU genes can be expressed from an internal promoter. Deletion anal ysis of the regulatory region upstream of norC indicated that a sequen ce motif which has identity to a motif in the gene encoding nitrite re ductase in strain 2.4.3 is critical for nor operon expression. Regulat ory studies demonstrated that the first four genes, norCBQD, are expre ssed only when the oxygen concentration is low and nitrate is present but that the two distal genes, nnrTU, are expressed constitutively.