Abstract: The enzyme nitrogenase, which catalyzes nitrogen fixation in Azotobacter vinelandii, consists of two components, the Fe protein and the MoFe protein. The MoFe protein is an α2β2 heterotetramer encoded by the nifD and nifK genes respectively. NifD, which consists of 492 amino acids, contains the FeMo cofactor; while the 523 amino acid β-subunit NifK is involved in electron transfer and protein-protein interaction. NifK contains several highly conserved residues implicated in its functions throughout the protein. However, the carboxyl terminus of NifK is not implicated in any of the known functions of this protein. Therefore, the present study explored the role of carboxyl terminal region of NifK by constructing a truncated NifK in which the last 58 amino acids were deleted by insertion of a stop codon at position 465. The results of growth analysis in standard Burke`s nitrogen media showed that the mutant strain exhibited a growth pattern similar to that of the wild type strain. However, when the media was adjusted to be slightly acidic, the strain that expressed the mutated NifK yielded a 15% lower growth compared to the wild type. These observations implied that the carboxyl terminus of the NifK contributes to the formation of a stable nitrogenase complex when A. vinelandii is grown in acidic environment.