Tn 5 Induced Symbiotic Mutants of Rhizobium meliloti 2011

Mutants of Rhizobium meliloti strain 2011 were induced by general Tn5 transposition using pBR325-Mob-Tn5 as a mobilising vector and inoculated on sterilized seedlings of Medicago sativa (alfalfa). After 3 or 4 weeks plants were examined for nodulation and also tested by the acetylene reduction assay. Of about 2400 mutants screened by this procedure, 23 showed defects or significant morphological and physiological changes in their symbiotic properties. In total, nine mutants failed to induce nodule formation. Among these there were five auxotrophic mutants (two for leucine, two for valine/isoleucine and one mutant with two Tn_5 insertions but unknown auxotrophic markers). Further auxotrophic mutants (arginine, adenine and cysteine) induced small nodules which sometimes were tumorlike in their appearance. These mutants did not fix nitrogen. Among the ineffective mutants, there were three prototrophic ones with an altered colony type.