DNA Homology and Taxonomy of Pseudomonas and Xanthomonas

Abstract

The relatedness between several nomen species of the Pseudo-monas-Xanthomonas group and some other organisms was numerically fixed throughdeoxyribonucleic acid (DNA) homology and DNA base composition. For Pseudomonas the numerically analyzed strains proposed by Ly -senko as neotypes were used. The mean % guanine+cytosine (G+C) was in the range 60-67.5; C14-DNA from either Pseudomonas fluorescens or P. putida was hybridized with DNA from 17 different species centers and the DNA homology was in the range 50-100%. Genetic species differentiation in the genus Pseudomonas seems justified. In three border cases (P. iodinum, P. diminuta and P. atlantica)DNA homology was only 28-50%, so the inclusion of these organisms in the genus Pseudomonas is uncertain. The species centers P. pavonacea and P. rubescens are omitted from the genus Pseudomonas because of their very low DNA homology and aberrant DNA base composition. Twenty-eight nomen species of Xanthomonas all form a narrow group in the range 63.5-69% (G+C). With two exceptions DNA homology with a median strain Xanthomonas pelargonii was always over 75% and frequently nearly complete; C14-DNA from P. fluorescens hybridized with Xanthomonas -DNA to the same extent as with the pseudomonads proper. The Xanthomonas cluster overlapped perfectly with part of the Pseudomonas group. It is proposed therefore to gather all xanthomonads in a single genetic species P. campestris. This is such a dense cluster that the preservation of separate species names for the border cases seems undesirable. About one half to two-thirds of Pseudomonas- and Xanthomonas -DNA is identical. The genera Rhizobium, Azotobacter and Azomonas appeared to be rather closely related to Pseudomonas since they shared some 40-50% of their DNA. The genus Serratia appeared to be more closely related to Pseudomonas than to Escherichia. The genera Gluconobacter, Acetobacter, Serratia and Escherichia shared some 20-30% DNA with Pseudomonas, but Bacillus -DNA was almost entirely different. From a comparison between DNA homology and taximetric similarity, it appeared that most pseudomonads would not contain unused genes. The advantages of a classification based on % (G+C) and DNA homology are obvious.