We hybridized 23S 2-C-labeled ribosomal ribonucleic acids (rRNAs) from type strains ATCC 13525, ATCC 15668, NCPPB 325, and NCPPB 528 with deoxyribonucleic acids (DNAs) from 65 strains, 23 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 in , 8th ed. The genus Migula 1894, 237 was divided into three large, distinct groups. The rRNA branch contains , and . The rRNA branch contains , and . The third rRNA branch contains , and . Each of these rRNA branches is as heterogeneous as a genus. The and rRNA branches are about as far removed from each other as they are from the genera and and the authentic genus These branches are members of the third rRNA superfamily. The rRNA branch is quite different, as it is a member of the second rRNA superfamily, which also contains , and some other genera. Along with data from rRNA hybridizations involving many different gram-negative taxa, these results show clearly that the three rRNA branches differ at least at the genus level. The genus is separate in its own right. It constitutes a very tight cluster consisting of , and ( covers older species names no longer in use). has rRNA cistrons that are indistinguishable from the rRNA cistrons of the xanthomonads mentioned above. There are a number of misnamed taxa. is a somewhat unusual member of ; likewise, and are not members of the genus , and is definitely not a member of the genus . 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 T (temperature of optimal renaturation) was meaningful only in the top 7 to 8°C T (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 from ribosomal DNA similarity corresponded to roughly 14% DNA homology).


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