Summary: Rhizobium phaseoli strain 1233 forms nitrogen-fixing nodules on and produces a brown pigment (probably melanin) in plate culture. This strain contains two plasmids of molecular weight about 200 × 10. Spontaneous deletions in the smaller plasmid abolished pigment production and the ability to nodulate beans, suggesting that genes involved in the determination of both these phenotypes are on this plasmid.

The conjugative plasmid pJB5JI, which carries genes that determine the ability to nodulate peas, was transferred from strain T3 into R. phaseoli strain 1233. The majority (about 97%) of transconjugants produced pigment and nodulated both peas and beans poorly. The others (about 3%) had received not only pJB5JI but also an extra plasmid present in strain T3 and they had lost the seller of the two plasmids of strain 1233, suggesting incompatibility between the plasmid transferred from T3 and that lost from the recipient. These strains did not produce pigment nor did they form nitrogen-fixing nodules on beans, but they nodulated peas as well as did strain T3.

Most (about 96%) of the bacteria isolated from nodules following inoculation of peas by the pigmented transconjugants failed to produce pigment. These strains contained the larger plasmid from strain 1233, and pJB5JI, but not the smaller plasmid of strain 1233; they behaved like in inducing profuse nitrogen-fixing nodules on peas. The few pigment-producing bacteria isolated from pea nodules had suffered a deletion in the smaller plasmid of strain 1233; they nodulated peas poorly but could not form nitrogen-fixing nodules on beans.


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