Rhizobium (Sinorhizobium) meliloti strains capable of dissimilatory nitrous oxide reduction (Nos+) carry a nosRZDFY gene cluster on a 10.1 kb EcoRI fragment of the nod megaplasmid near the fixGHIS genes. These nos genes are arranged in three complementation groups and the 10.1 kb EcoRI fragment is sufficient to confer Nos activity to R. meliloti strains lacking such activity. An overlapping HindIII fragment containing the nosRZDFY genes but missing a 0-6 kb HindIII-EcoRI downstream segment was found incapable of imparting Nos activity to strains unable to reduce nitrous oxide, suggesting the presence of other nos gene(s) in this region. Tn5 introduced near the HindIII site resulted in mutants with a Nos- phenotype. Complete sequence analysis of nosY showed that it was well-conserved with respect to that of Pseudomonas stutzeri. Two previously unreported genes downstream of nosY in R. meliloti were also revealed. Contiguous with nosY was a sequence showing 63% identity with the ORFL protein of P. stutzeri. It appeared to be in the same operon as nosDFY and was predicted to encode a membrane lipoprotein similar to the putative NosL of P. stutzeri. Unlike the latter protein, however, amino acid sequences typical of metal-binding sites and cysteine residues indicative of the active site of protein disulphide isomerase were absent in the predicted NosL of R. mellioti. The Tn5 mutations resulting in a Nos- phenotype were localized within a 966 nucleotide gene 31 nucleotides downstream of nosDFYL with the same orientation. The new gene, nosX, was determined to be in a separate complementation group. It encoded a periplasmic protein with homology in the C-terminal domain with Rnff of Rhodobacter capsulatus and with a hypothetical Escherichia coli protein, YOJK. It was concluded that there are seven genes constituting the nos cluster in R. meliloti. They are organized in four complementation groups and in the same orientation, spanning a distance of about 9 kb on the nod megaplasmid.
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