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J. Bacteriol., Jul 1997, 4172-4178, Vol 179, No. 13
T Hurek, T Egener and B Reinhold-Hurek
Nitrogenase is a functionally constant protein catalyzing N2 reduction,
which is found in many phylogenetic lineages of Archaea and Bacteria. A
phylogenetic analysis of nif genes may provide insights into the evolution
of the bacterial genomes. Moreover, it may be used to study diazotrophic
communities, when classical isolation techniques may fail to detect all
contributing populations. Among six species of the genus Azoarcus,
diazotrophic Proteobacteria of the beta subclass, the deduced amino acid
sequences of nifH genes of two species were unusually divergent from each
other. Nitrogenases of the "authentic" Azoarcus branch formed a
monophyletic unit with those of gamma Proteobacteria, thus being in
accordance with 16S ribosomal DNA phylogeny. The nitrogenase proteins of
the two aberrant strains clustered within the alpha proteobacterial clade
with rhizobial nitrogenases. This relationship was supported by bootstrap
values of 87 to 98% obtained by various distance and maximum parsimony
methods. Phylogenetic distances of NifH proteins indicate a possible
lateral gene transfer of nif genes to Azoarcus from a common donor of the
alpha subclass at the time of species diversification or several more
recent, independent transfers. Application of the phylogenetic analysis to
DNA isolated from environmental samples demonstrated novel habitats for
Azoarcus: in guts of termites and rice grown in Japan, nifH genes belonging
to the authentic Azoarcus branch were detected. This is the first evidence
suggesting the occurrence of Azoarcus spp. in a plant other than its
originally described host, Kallar grass. Moreover, evidence for expression
of nif genes inside grass roots was obtained by in situ hybridization
studies with antisense nifH probes.
Copyright © 1997, American Society for Microbiology
Divergence in nitrogenases of Azoarcus spp., Proteobacteria of the beta subclass
Max-Planck-Institut fur terrestrische Mikrobiologie, Arbeitsgruppe Symbioseforschung, Marburg, Germany.
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