We prepared hybrids between C-labeled ribosomal ribonucleic acid (rRNA) from either NCTC 9757 or NCTC 9796 and deoxyribonucleic acid (DNA) from a great variety of named gram-negative bacteria, including many type and reference strains. Each hybrid was described by two parameters: (i) , the temperature at which 50% of the hybrid was denatured; (ii) the percent rRNA binding, the amount (micrograms) of [C]rRNA duplexed in stringent conditions per 100 μg of filter-fixed homologous or heterologous DNA. Each taxon occupied a definite area on the rRNA similarity maps. All and strains formed tight separate clusters around their neotype strains. The rRNA parameters of both taxa were about as different from each other as was the case for many genera. Both taxa are phenotypically and genotypically (by DNA-DNA hybridization) likewise very different. We proposed to elevate each cluster to genus rank as and The following strains were misnamed and belonged in neither genus: “ NCTC 10590 and 10591; “” GA; “ ATCC 17056; “” (“”) ATCC 9897, 15728, and 15729; “ (now ) NCTC 2847; and “ (now sp.) NCTC 2416. The rRNA cistrons of and resembled most closely those of section II and III, the authentic , the H-oxidizing and , and NCIB 8193. These taxa displayed a number of phenotypical similarities. We suggested that all these taxa are the closest taxonomic relatives of and


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