A homolog of theRhizobium meliloti nitrogen fixation genefixN is involved in the production of a microaerobically induced oxidase activity in the phytopathogenic bacteriumAgrobacterium tumefaciens

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Abstract
Hybridization analysis using theRhizobium meliloti nitrogen fixation genefixN as a probe revealed the presence of a homologous DNA region in the phytopathogenic bacteriumAgrobacterium tumefaciens. Hybridization signals were also detected with total DNAs ofRhizobium leguminosarum bv.phaseoli, Rhodobacter capsulatus andEscherichia coli, but not those ofXanthomonas campestris pv.campestris andPseudomonas putida. The hybridizing fragment fromA. tumefaciens was cloned and sequenced. The predicted gene product of one of the two open reading frames identified on the sequenced fragment shows homology to FixN of differentRhizobiaceae as well as a low but significant similarity to subunit I of heme copper oxidases from various bacteria. The presence of five strictly conserved histidine residues previously implicated in forming ligands to heme and CuB in oxidases and the predicted membrane topology provide evidence that theA. tumefaciens fixN-like gene product is a component of the heme copper oxidase superfamily. The incomplete open reading frame starting only 8 nucleotides downstream of thefixN-like gene exhibits homology toRhizobium fixO. Using anuidA (GUS) gene fusion it could be shown that theA. tumefaciens fixN-like gene is preferentially expressed under microaerobic conditions. Expression of theuidA fusion is abolished inR. meliloti fixJ andfixK mutants, indicating that an Fnr-like protein is involved in transcriptional regulation of thefixN-like gene in A.tumefaciens. The presence of an upstream DNA sequence motif identical to the Fnr-consensus binding site (anaerobox) further supports this hypothesis.A. tumefaciens mutated in thefixN-like gene shows decreased TMPD-specific oxidase activity under microaerobic conditions, indicating that thefixN-like gene or operon codes for proteins involved in respiration under reduced oxygen availability.