Symbiotic properties of some auxotrophic mutants of Rhizobium meliloti and of their prototrophic revertants

Summary 1. Auxotrophic mutants of 2 strains ofRhizobium meliloti have been isolated: among them 4 are purine or pyrimidine-dependent (adenine, cytosine), 4 are sulfur amino acid-dependent (methionine, cysteine) and 3 glycine-dependent. 2. All of these mutants induce nodule formation on lucerne (variety ‘Du Puits’). 3. There are associations between nutritional requirements and effectiveness in nitrogen fixation on this host. All the amino acid-dependent mutants are effective. Glycine-dependent mutants are more effective than the wild-type strain, whereas cyste?ne and methionine-dependent mutants are as effective as the wild-type strain. On the other hand, purine and pyrimidine-dependent mutants are ineffective. 4. All the prototrophic revertants of ineffective auxotrophic strains are effective. Some of them are more effective than wild-type strain.