Summary auto-generated
This study re-examines the taxonomic classification of Azotobacter vinelandii, a free-living soil bacterium capable of nitrogen fixation in aerobic conditions. Although currently classified in the family Pseudomonadaceae, the authors investigated whether A. vinelandii should actually be reclassified as a Pseudomonas species. Using 16S rRNA gene sequencing and analysis of 25 housekeeping protein-encoding genes, they found that A. vinelandii clusters closely with Pseudomonas aeruginosa and other Pseudomonas species. Additional evidence includes shared HaeIII RFLP patterns, conserved repeating elements in 16S rRNA genes, and possession of OprF, a diagnostic outer-membrane protein previously found only in Pseudomonas sensu stricto. The phylogenetic trees generated from both individual housekeeping genes and concatenated protein sequences showed identical topology to 16S rRNA-based trees. Furthermore, A. vinelandii harbors an OprF orthologue and alginate biosynthesis genes (AlgX) found only in Pseudomonas species. The authors conclude that A. vinelandii belongs phylogenetically within genus Pseudomonas sensu stricto, suggesting nitrogen-fixing capacity was acquired through lateral gene transfer rather than representing a fundamental taxonomic distinction.
Key findings
- 16S rRNA gene sequences and housekeeping protein analyses place A. vinelandii within the Pseudomonas aeruginosa clade rather than as a separate genus
- A. vinelandii possesses OprF and AlgX orthologues, diagnostic proteins previously identified only in Pseudomonas sensu stricto species
- Phylogenetic analysis of 25 housekeeping proteins and concatenated sequences produces trees with identical topology to 16S rRNA-based phylogenies, strongly supporting A. vinelandii's reclassification as Pseudomonas
- Nitrogen fixation capability, traditionally used to distinguish A. vinelandii from Pseudomonas, likely arose through lateral gene transfer rather than representing a fundamental taxonomic difference
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Abstract
Azotobacter vinelandii is a widely distributed free-living soil bacterium. This Gram-negative, strictly aerobic bacterium has many interesting features, including the ability to grow on a wide variety of carbohydrates, alcohols and organic acids, alginate production and nitrogen fixation. A. vinelandii synthesizes three different types of nitrogenase enzymes. Unlike most diazotrophs, A. vinelandii is able to fix nitrogen in the presence of atmospheric oxygen concentrations. To protect its nitrogenase enzymes from oxygen inactivation, this bacterium is equipped with particular physiological mechanisms, such as a high respiration rate. Currently, Azotobacter is classified as a genus of the family Pseudomonadaceae ().
Using in vivo expression technology (IVET), we screened a nitrogen-fixing Pseudomonas stutzeri strain for genes specifically expressed in rice rhizosphere (Rediers et al., 2003). Sequence analysis of these genes revealed high sequence similarity not only to genes of several other Pseudomonas strains but also to A. vinelandii genes. These observations and the availability of a (draft) genome sequence of A. vinelandii and several Pseudomonas species urged us to inspect the phylogenetic relationship of A. vinelandii to Pseudomonas sensu stricto.