Biological nitrogen fixation: primary structure of the Klebsiella pneumoniae nifH and nifD genes.

A DNA fragment carrying the Klebsiella pneumoniae nifK, D, and H genes was isolated from the nif- strain UNF841 (Tn5::nifK) by molecular cloning into the Escherichia coli plasmid pBR325. The nucleotide sequences of both the nifH gene, which encodes the Fe protein of the nitrogenase enzyme complex, and 622 nucleotides of the nifD gene, which encodes the alpha-subunit of the Mo-Fe protein, were determined by direct DNA sequencing by both the chemical and chain termination methods. A comparison of the primary structure of the Klebsiella nifH gene and its product with that recently determined for the blue-green alga Anabaena demonstrates that the gene sequences are more divergent than the protein sequence data would suggest. This implies that despite the strong, presumably functional, constraints that act at the protein structure level, the nucleotide sequence of the gene and its mRNA are only restrained by the coding requirements, allowing substantial drift in codon usage.