Rhizobium japonicum derivatives differing in nitrogen-fixing efficiency and carbohydrate utilization.

Four derivatives of Rhizobium japonicum 110 were isolated on the basis of morphologically different colonies formed on yeast extract-mannitol-HM salts medium. All are able to nodulate Lee soybeans. The bacteria-plant associations formed by each clone have measurable acetylene-reducing activity, but those formed by two of these clones (designated L1-110 and L2-110) are 5- to 10-fold less efficient than those formed by the others (designated I-110 and S-110). These derivatives were not detectable with ordinary culture techniques since, because of cell adherence, genetically mixed colonies result. When a detergent (Tween 40 at 0.01%, vol/vol) was added to the dilution medium, separate clones resulted. The metabolic basis for the gross differences in colony morphology on yeast extract-mannitol-HM salts medium was found to be that L1-110 and L2-110 utilized p-mannitol for growth, whereas I-110 and S-110 did not. These clones differ analogously in ability to utilize D-arabitol. Clones I-110 and L1-110 were chosen for studies of growth rates on various carbohydrates. Although clone I-110 and clone L1-110 did not differ in growth rates on a number of sugars, such as gluconate, arabinose, glycerol, and mannose, they differed in growth rates on glucose and fructose. Although clone I-110 grew faster on glucose than did clone L1-110, clone L1-110 grew faster on fructose than did clone I-110. Clones I-110 and L1-110 showed identical responses to several antibiotics and deoxyribonucleic acid (DNA) synthesis inhibitors and identical susceptibility to some highly specific bacteriophages. Identical buoyant densities of their DNAs in isopycnic CsCl density gradients and identical thermal denaturation temperatures of their DNAs suggest that clones I-110 and L1-110 have the same DNA base composition. Preliminary DNA/DNA hybridization experiments show that strain I-110 DNA and strain L1-110 DNA have a high degree of common polynucleotide sequences.