Abstract The complete nucleotide sequence (24,206 base-pairs) of the Klebsiella pneumoniae gene region for nitrogen fixation ( nif) is presented. Coding regions corresponding to the 19 known nif genes (including nifW and nifZ) could be identified. An additional open reading frame of 216 base-pairs, called nifT, was detected between nifK and nifY. Search for transcriptional signal structures revealed some unusual features: (1) several possible NifA-binding motifs are present in the intergenic regions between nifJ and nifH as well as between nifX and nifU; (2) a perfect NifA-binding motif, preceding the nifENX promoter, is located within an inverted repeat structure; (3) structures resembling the consensus nif promoter are found within the coding regions of nifW and nifZ and, together with a NifA-binding motif, in nifN. Typical rho-independent termination structures were detected only downstream from the nifHDKTY and the nifBQ operons. Analysis of the deduced amino acid sequences revealed the presence of two Cys-X 2-Cys-X 2-Cys-X 3-Cys-Pro clusters in the pyruvateflavodoxin oxidoreductase NifJ. This arrangement of cysteine residues is normally present only in ferredoxins. A high degree of homology between the two gene products (NifE and NifN) involved in iron-molybdenum cofactor biosynthesis and the two nitrogenase component I structural proteins (NifD and NifK) was found. All four proteins are characterized by the conserved motif His-Gly-X 2-Gly-Cys, which may play a role in binding the iron-molybdenum cofactor.