Differential Expression of the Pea Symbiotic Plasmid pJB5JI in Genetically Dissimilar Backgrounds

The Rhizobium leguminosarum h o s t-range (Sym) plasmid, pJB5JI, is a 130 Mdal conjugative plasmid which carries the genes for host-specificity (i.e., for peas) and some of the genes for ni trogen-fixation. We have t ransferred plasmid pJB5JI into different species of Rhizobium and examined the symbiotic propert ies of the result ing transconjugants. The donor strain R. leguminosarum 6015 (pJB5JI), was only moderately effective on peas. Transconjugants of a Nod mutant of R. trifolii, #2, became highly effective on peas, but did not regain their ability to nodulate clover. Transconjugants of a Nod strain of R. trifolii, 0403, could nodulate peas, but only ineffectively, and retained the abil i ty to nodulate clover effectively. All reisolates of #2 transconjugants from pea nodules had gained the abil i ty to form ineffective nodules on clover. Reisolates of 0403 transconjugants from both clover and pea nodules exhibited three different symbiotic phenotypes: They could n odulate (1) both peas and clover effectively; (2) clover effectively but peas ineffectively; or (3) only clover effectively. Transconjugants made with a fastgrowing soybeanRhizobium were unable to nodulate peas, but retained the abi l i ty to nodulate soybeans. The presence of the plasmid in these t ransconjugants was ascertained by backcrossing to a Nod of R. leguminosarum..In all of the studies, the identi ty of the donors, the recipients and the t ransconju gants were confirmed by their antibiotic and auxotrophic markers, as well as by immunofluorescence and immunodiffusion using strain specific antisera. In this study, we have shown that plasmid pJB5JI is readily transferable to different species of Rhizobium. However, i ts symbiotic expression varies depending on the genetic background i t resides in.