Sequence of the nifD gene coding for the α subunit of dinitrogenase from the cyanobacterium Anabaena

Abstract
The nucleotide sequence of nifD, the structural gene for the .alpha. subunit of dinitrogenase from Anabaena 7120, was determined. The coding sequence contains 1440 nucleotides, which predict an amino acid sequence of 480 residues and MW of 54,283. The predicted sequence contains 8 cysteines, of which 5 are conserved with respect to adjoining sequences and position relative to the .alpha. subunits of dinitrogenase from Azotobacter, Clostridium and Klebsiella. Because there are also 5 conserved cysteines in the .beta. subunit of Anabaena dinitrogenase, the number of cysteine residues participating as ligands to FeS clusters is likely to be 20 per .alpha.2.beta.2 tetramer. This number is sufficient to accommodate the known 4 Fe4S4 clusters, leaving at least 4 cysteines to be shared among the 2 FeMo cofactors and the more poorly characterized two-iron center. Although the .alpha.- and .beta.-subunit gene sequences are not recognizably homologous, their secondary structures, predicted from the sequences, indicate similar domains around 3 of the conserved cysteine residues.

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