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Second gene ( nif H * ) coding for a nitrogenase iron protein in Azotobacter chroococcum is adjacent to a gene coding for a ferredoxin‐like protein
Author(s) -
Robson Robert,
Woodley Paul,
Jones Robert
Publication year - 1986
Publication title -
the embo journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 7.484
H-Index - 392
eISSN - 1460-2075
pISSN - 0261-4189
DOI - 10.1002/j.1460-2075.1986.tb04341.x
Subject(s) - azotobacter chroococcum , biology , ferredoxin , nitrogenase , gene , azotobacteraceae , biochemistry , azotobacter , nitrogen fixation , genetics , enzyme , bacteria
Azotobacter chroococcum MCD1 contains a cluster of nitrogen fixation ( nif ) genes coding for the structural polypeptides for nitrogenase ( nif H for the Fe‐protein and nif D and nif K for the MoFe protein) and a second sequence in the genome homologous to nif H. DNA fragments bearing this second nif H‐like sequence were cloned and the DNA sequence around the homologous region determined. Two open reading frames were identified in this region. One codes for a protein of 289 amino acid residues and is highly homologous to other Fe‐proteins but is different from the gene adjacent to the nif DK genes in A. chroococcum . This putative gene we call nif H * . The following open reading frame codes for a protein of 63 amino acids, nine of which are cysteine residues. The protein is homologous to the small low‐potential ferredoxins found in anaerobic bacteria, and in particular those from Chlorobium limicola . Linkage between a structural gene for nitrogenase and a small ferredoxin has not previously been observed. Sequence analysis suggests that the two genes form an operon. Transcription of the ferredoxin gene on a 1320‐bp transcript was only detectable under conditions in which A. chroococcum MCD1155, which carries a chromosomal deletion of 6.3 kb removing the entire nif HDK cluster, is capable of fixing N 2 , i.e. in media containing no added molybdenum or high levels of NH 3 . The size of the observed transcript agrees well with the predicted size for a transcript encoding nif H * and the ferredoxin genes. Expression of the nif H * promoter was not significantly activated in Escherichia coli even when nif A, the positive activator of nif genes in Klebsiella pneumoniae , was supplied in multiple copies. The results are discussed in relation to an alternative pathway for N 2 fixation in A. chroococcum .