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Probable synonymy of the nitrogen-fixing genus Azotobacter and the genus Pseudomonas

Landcare Research, Private Bag 92170, Auckland, New Zealand

Correspondence
J. M. Young
youngj{at}LandcareResearch.co.nz

The relationships of the genus Azotobacter, Azomonas macrocytogenes and the genus Pseudomonas were revealed by comparative analysis of partial 16S rRNA and atpD, carA and recA gene sequences and as concatenated nucleotide and peptide sequences. Sequence similarities of Azotobacter species and Azomonas macrocytogenes indicated that these may be considered to be synonyms at the molecular level. In addition, these species show an intimate relationship with species of Pseudomonas, especially P. aeruginosa (the type species of the genus). In terms of the current circumscription of the genus Pseudomonas, Azotobacter and Azomonas macrocytogenes should be considered for amalgamation with Pseudomonas. Azotobacter and Azomonas comprise nitrogen-fixing strains with large pleomorphic cells that form cysts, and peritrichous flagella insertion; characteristics not included in the current circumscription of Pseudomonas. The data are discussed in the light of whether lateral transfer of genes could be involved in the determination of significant morphological characteristics, thus leading to a problem that may be encountered more frequently: how to resolve classification of taxa based on conserved sequences with those based on their phenotype. More fundamentally, the results illuminate problems that will increasingly be encountered: by what criteria can taxa be delineated, what are the most appropriate methods for classification, and what are the proper assumptions of bacterial classification?

The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA, atpD, carA and recA gene sequences determined in this study are given in Fig. 1 and Supplementary Figs S1 and S2.

All authority references for names can be found at Euzéby (1997).

Figures showing the comparative analysis of individual atpD, carA and recA nucleotide and peptide sequences of Azomonas, Azotobacter and Pseudomonas species using the maximum-likelihood and neighbour-joining algorithms are available as supplementary material with the online version of this paper.