A Study of Some Mutations in a Strain of Rhizobium trifolii

SUMMARY: Mutants of a strain of Rhizobium trifolii were obtained by selective action of bacteriophage and streptomycin and by exposure to ultraviolet (u.v.) radiation. All mutants resistant to bacteriophage were streptomycin-susceptible and all resistant to streptomycin were bacteriophage-susceptible; no survivor after exposure to u.v. radiation resisted either the bacteriophage or streptomycin. Mutation to bacteriophage resistance was closely correlated with inability to fix nitrogen in symbiosis with red clover. Some streptomycin-resistant mutants used streptomycin as a nutrient supplement, enabling them to grow on a mineral medium without growth factors. All but two mutants remained serologically indistinguishable from the original strain.