SUMMARY: There is a correlation between the base composition of pure DNA from 35 strains of Rhizobium and their type of flagellation. There are two groups of Rhizobia. The peritrichously flagellated organisms, which usually grow fast, have a low % (guanine + cytosine) composition in the range 58·6–63·1%; these organisms occur in all cross-inoculation groups investigated. From a comparison with literature data it appears that this group is constituted by two subgroups: and The subpolarly flagellated, slow-growing strains have a some-what higher (guanine + cytosine) content, mostly in the range 62·8–65·5%; these organisms appear to be specialized mainly for the lupin, soybean, cowpea, Lotus, Wistaria and Robinia groups, It is proposed that only one genetic species is involved, to be called The rhizobia would thus contain only three genetic species. The relationship between the high and low (guanine + cytosine) groups is stressed by their closely related % (G + C) values, the near-identity of the average compositional distribution of the DNA molecules and because the peritrichous organisms not infrequently show a subpolar flagellum.


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