IDENTIFICATION OF A RHIZOSPHERE PROTEIN ENCODED BY THE SYMBIOTIC PLASMID OF RHIZOBIUM-LEGUMINOSARUM

A protein was identified which was made by wild-type strains of R. leguminosarum but not by nodulation-deficient derivatives which had deletions of their symbiotic plasmids. The protein, which had a subunit MW of .apprx. 24,000 (24K), was present in large amounts within bacteria that reisolated from the surface of inoculated pea roots but was not detected in bacteroids isolated from nodules. The protein could also be induced during growth of R. leguminosarum on nutrient medium and was purified from the cytoplasmic fraction of broken cells. Antiserum raised against the purified protein was used to screen transposon-induced mutants of R. leguminosarum and 4 independent mutants were isolated which lacked the protein. The sites of the Tn5 insertions mapped between the nitrogenase and nodulation genes on symbiotic plasmid pRL1JI, .apprx. 5 kilobases (kb) from the nitrogenase genes and 13 kb from the nodulation genes. Genetic determinants for the 24K protein were closely linked to plasmid-borne nodulation genes for all strains of R. leguminosarum tested. The mutants which lacked the 24K protein still formed normal N2-fixing nodules on peas, and the function of the protein is unknown.