Nucleotide sequence and genetic analysis of the nifB-nifQ region from Azotobacter vinelandii

A 3.8-kilobase-pair EcoRI fragment which corrects the mutations carried by the NifB- Azotobacter vinelandii strains CA30 and UW45 was cloned, and its nucleotide sequence was determined. Four complete open reading frames (ORFs) and two partial ORFs were found. The translation product of the first partial ORF is the carboxy-terminal end of a protein homologous to the nifA gene product from Klebsiella pneumoniae. A 285-base-pair sequence containing a potential nif promoter and nif regulatory sites separates this nifA gene from the first complete ORF which encodes a protein homologous to nifB gene products from K. pneumoniae and Rhizobium species. The Tn5 insertion in strain CA30 and the nif-45 mutation of strain UW45 are located within this nifB gene. The ORF downstream from nifB predicts an amino acid sequence with a cysteine residue pattern that is characteristic of ferredoxins. No similarities were found between the translation product of the third complete ORF and those of nif genes from other organisms. At the carboxy-terminal end of the predicted translation product of the fourth complete ORF, 30 of 60 amino acid residues were identical with the sequence of the nifQ gene product from K. pneumoniae. The partial ORF located at the end of the fragment encodes the N-terminal part of a potential protein with an unknown function. Northern (RNA) blot analysis indicated that transcripts from the region containing the four complete ORFs were NH4+ repressible and that the transcription products were identical in cells derepressed under conditions of Mo sufficiency or Mo deficiency or in the presence of vanadium. In contrast to the NifB- strain CA30, which is Nif- under all conditions, mutants that carry mutations affecting the C-terminal end of nifB or genes located immediately downstream from nifB, grew under all N2-fixing conditions. However, in the presence of Mo, most of the strains required 1,000 times the amount of molybdate that is sufficient for maximal growth of the wild-type strain CA under N2-fixing conditions. Growth data from strain CA37, which carries a Kanr insertion in nifQ, indicate that nifQ in A. vinelandii is not required for N2 fixation in the presence of V2O5 or under Mo-deficient conditions. Growth studies and acetylene reduction assays performed on two nifEN deletion strains showed that nifE and nifN are required for N2 fixation under Mo sufficiency, as previously observed (K. E. Brigle, M. C. Weiss, W. E. Newton, and D. R. Dean, J. Bacteriol. 169:1547-1553, 1987), but not under conditions of Mo deficiency or in the presence of 50 nM V2O5.