Molecular and General Genetics MGG

, Volume 216, Issue 2–3, pp 353–363 | Cite as

DNA sequence and genetic analysis of the Rhodobacter capsulatus nifENX gene region: Homology between NifX and NifB suggests involvement of NifX in processing of the iron-molybdenum cofactor

  • Conrado Moreno-Vivian
  • Manfred Schmehl
  • Bernd Masepohl
  • Walter Arnold
  • Werner Klipp


Rhodobacter capsulatus genes homologous to Klebsiella pneumoniae nifE, nifN and nifX were identified by DNA sequence analysis of a 4282 bp fragment of nif region A. Four open reading frames coding for a 51188 (NifE), a 49459 (NifN), a 17459 (NifX) and a 17472 (ORF4) dalton protein were detected. A typical NifA activated consensus promoter and two imperfect putative NifA binding sites were located in the 377 bp sequence in front of the nifE coding region. Comparison of the deduced amino acid sequences of R. capsulatus NifE and NifN revealed homologies not only to analogous gene products of other organisms but also to the α and β subunits of the nitrogenase iron-molybdenum protein. In addition, the R. capsulatus nifE and nifN proteins shared considerable homology with each other. The map position of nifX downstream of nifEN corresponded in R. capsulatus and K. pneumoniae and the deduced molecular weights of both proteins were nearly identical. Nevertheless, R. capsulatus NifX was more related to the C-terminal end of NifY from K. pneumoniae than to NifX. A small domain of approximately 33 amino acid residues showing the highest degree of homology between NifY and NifX was also present in all nifB proteins analyzed so far. This homology indicated an evolutionary relationship of nifX, nifY and nifB and also suggested that NifX and NifY might play a role in maturation and/or stability of the iron-molybdenum cofactor. The open reading rame (ORF4) downstream of nifX in R. capsulatus is also present in Azotobacter vinelandii but not in K. pneumoniae. Interposon-induced insertion and deletion mutants proved that nifE and nifN were necessary for nitrogen fixation in R. capsulatus. In contrast, no essential role could be demonstrated for nifX and ORF4 whereas at least one gene downstream of ORF4 appeared to be important for nitrogen fixation.

Key words

Rhodobacter capsulatus Nitrogen fixation DNA sequence analysis nifE, nifN, nifX genes Protein comparisons 


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Copyright information

© Springer-Verlag 1989

Authors and Affiliations

  • Conrado Moreno-Vivian
    • 1
  • Manfred Schmehl
    • 1
  • Bernd Masepohl
    • 1
  • Walter Arnold
    • 1
  • Werner Klipp
    • 1
  1. 1.Lehrstuhl für Genetik, Fakultät für BiologieUniversität BielefeldBielefeld 1Federal Republic of Germany

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