Intra- and Intergeneric Similarities of Pseudomonas and Xanthomonas Ribosomal Ribonucleic Acid Cistrons

Abstract

We hybridized 23s 2- 14C-labeled ribosomal ribonucleic acids (rRNAs) from type strains Pseudomonas fluorescens ATCC 13525, Pseudomonas acidovorans ATCC 15668, Pseudomonas solanacearum NCPPB 325, and Xanthomonas campestris NCPPB 528 with deoxyribonucleic acids (DNAs) from 65 Pseudomonus strains, 23 Xanthomonas strains, and 148 mostly gram-negative strains belonging to 43 genera and 93 species and subspecies including more than 60 type strains. Our findings confirm and extend the findings derived from ribonucleic acid hybridizations by the Berkeley group, but differed in some respects from the groupings of Pseudomonas in Bergey 's Manual of Determinative Bacteriology, 8th ed. The genus Pseudomonas Migula 1894, 237 was divided into three large, distinct groups. The PseudomonasJIuorescens rRNA branch contains Pseudomonus aeruginosa, Pseudomonas fluorescens, Pseudomonas chlororaphis, Pseudomonas aureofaciens, Pseudomonas syringae, Pseudomonas putida, Pseudomonas stutzeri, Pseudomonas mendocina, Pseudomonas cichorii, Pseudomonas alcaligenes, and Pseudomonas pseudoalcaligenes. The Pseudomonas acidovorans rRNA branch contains Pseudomonas acidovorans, Pseudomonas testosteroni, Pseudomonas delajieldii, Pseudomonas facilis, Pseudomonas palleronii, Pseudomonas saccharophila, and Pseudomonas flava. The third rRNA branch contains Pseudomonas solanacearum, Pseudomonas cepacia, Pseudomonas marginata, Pseudomonas caryophylli, and Pseudomonas lemoignei. Each of these rRNA branches is as heterogeneous as a genus. The Pseudomonas solanacearum and Pseudomonas acidovorans rRNA branches are about as far removed from each other as they are from the genera Janthinobacterium and Derxia and the authentic genus Alcaligenes. These branches are members of the third rRNA superfamily. The Pseudomonas fluorescens rRNA branch is quite different, as it is a member of the second rRNA superfamily, which also contains Azotobacter, Azomonas, Xanthomonas, and some other genera. Along with data from rRNA hybridizations involving. many different gram-negative taxa, these results show clearly that the three Pseudomonas rRNA branches differ at least at the genus level. The genus Xanthomonas is separate in its own right. It constitutes a very tight cluster consisting of Xanthomonas campestris, Xanthomonas fragariae, Xanthomonas axonopodis, and Xanthomonas albilineans (Xanthomonas campestris covers older species names no longer in use). Xanthomonas (Aplanobacter) populi has rRNA cistrons that are indistinguishable from the rRNA cistrons of the xanthomonads mentioned above. There are a number of misnamed taxa. Pseudomonas maltophilia is a somewhat unusual member of Xanthomonas; likewise, Pseudomonas diminuta and Pseudomonas vesicularis are not members of the genus Pseudomonas, and Xanthomonas ampelina is definitely not a member of the genus Xanthomonas. The exact taxonomic positions of the latter three species are unknown. A quantitative comparison showed that fine differentiation of strains by means of DNA-DNA hybridization under stringent conditions at TOR (temperature of optimal renaturation) was meaningful only in the top 7 to 8°C Tm(c) (thermal elution temperature range, 73 to 81°C) of our DNA-rRNA similarity maps and dendrograms (a difference of 1°C in thermal elution temperature Tm(e) from ribosomal DNA similarity corresponded to roughly 14% DNA homology).