The levels of genetic relatedness of 19 strains which utilize one-carbon compounds were determined by deoxyribonucleic acid (DNA)-DNA hybridization in solution under optimal conditions (S1 nuclease technique). Most of these hyphomicrobia fell into four groups with high levels of relatedness (level of homology within each group, 86 to 110%). These groups were only distantly related to each other (levels of homology between groups, 1 to 9%). Three additional groups of C-utilizing hyphomicrobia were represented by only one strain each. In addition, the levels of DNA-DNA homology of four species and 11 -like isolates were determined. Seven of these isolates formed three groups containing two or three strains each; the level of homology within each group was 94 to 120%. These groups of peptide-utilizing strains were related to spp. at a DNA-DNA homology level of 13 to 43%; thus, they represented new species of Four of the isolates had less than 10% DNA homology with either reference strains or spp. Strain B-1408 was distinguished by its DNA base composition of 46.19 mol% guanine plus cytosine, which is 14 mol% below the average base composition of spp. or spp. The levels of genetic relatedness of other hyphal, budding bacteria, such as spp., genus F, , and genus T, to sp. strain MC-750 were too low to be evaluated by DNA-DNA hybridization techniques.


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