Abstract
BackgroundBiological nitrogen fixation is a prokaryotic process that plays an essential role in the global nitrogen cycle. Azorhizobium caulinodans ORS571 has the dual capacity to fix nitrogen both as free-living organism and in a symbiotic interaction with Sesbania rostrata. The host is a fast-growing, submergence-tolerant tropical legume on which A. caulinodans can efficiently induce nodule formation on the root system and on adventitious rootlets located on the stem.ResultsThe 5.37-Mb genome consists of a single circular chromosome with an overall average GC of 67% and numerous islands with varying GC contents. Most nodulation functions as well as a putative type-IV secretion system are found in a distinct symbiosis region. The genome contains a plethora of regulatory and transporter genes and many functions possibly involved in contacting a host. It potentially encodes 4717 proteins of which 96.3% have homologs and 3.7% are unique for A. caulinodans. Phylogenetic analyses show that the diazotroph Xanthobacter autotrophicus is the closest relative among the sequenced genomes, but the synteny between both genomes is very poor.ConclusionThe genome analysis reveals that A. caulinodans is a diazotroph that acquired the capacity to nodulate most probably through horizontal gene transfer of a complex symbiosis island. The genome contains numerous genes that reflect a strong adaptive and metabolic potential. These combined features and the availability of the annotated genome make A. caulinodans an attractive organism to explore symbiotic biological nitrogen fixation beyond leguminous plants.
Highlights
Biological nitrogen fixation is a prokaryotic process that plays an essential role in the global nitrogen cycle
SFnigapusrheo1t of the output generated after analysis of the A. caulinodans genome with the Genome Atlas tool Snapshot of the output generated after analysis of the A. caulinodans genome with the Genome Atlas tool
From the outer to the inner circle: circle 1, protein hits in the UNIPROT database; circle 2, synteny between 15 α-proteobacterial genomes; circle 3, Agrobacterium tumefaciens C58; circle 4, Xanthobacter autotrophicus Py2; circle 5, Sinorhizobium meliloti 1021; circle 6, Bradyrhizobium japonicum USDA 110; circle 7, Rhizobium leguminosarum 3841; circle 8, Rhizobium etli CFN42; circles 9 and 10, Mesorhizobium loti strains BNC1 and MAFF303099, respectively; circle 11, Nitrobacter winogradskyi Nb225; circles 12, 13, 14, 15, and 16, Rhodopseudomonas palustris strains CGA009, HaA2, BisA53, BisB18, and BisB5, respectively; circle 17, intrinsic curvature; circle 18, stacking energy; circle 19, position preference; circle 20, genome annotation; circle 21, global repeats; circle 22, inverted repeats; circle 23, GC skew; circle 24, percent AT
Summary
Biological nitrogen fixation is a prokaryotic process that plays an essential role in the global nitrogen cycle. Nitrogen-fixing bacteria can be divided in two major groups: free-living nitrogen fixers or diazotrophs that directly assimilate ammonia for growth and symbiotic nitrogen fixers that pass ammonia to a eukaryotic host and indirectly profit from nitrogen fixation by occupying a particular ecological niche or by supporting the population through better feeding. In the latter group, the symbiosis between leguminous crop plants and rhizobia is of great importance for agriculture. Stem and root nodulation are similar, in the latter the nodular vascular system is connected to that of the stem via the vascular bundles of the adventitious root primordium [3]
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