The pSym Plasmid of Rhizobium Meliloti . III. Expression of pSym Functions in Agrobacterium Tumefaciens

The pSym megaplasmid of R. meliloti 2011, mobilized by RP4, or pGMI42 wich carries a 290 Kb pSym fragment including nitrogenase and nod genes (described by Batut et al. in Poster I of this series), was introduced into Agrobacterium tumefaciens. The resulting transconjugants induced root deformations specifically on the homologous hosts Medicago sativa and Melilotus alba and not on the heterologous hosts Trifolium pratense and T. repens. The root deformations were shown to be genuine nodules by physiological and cytological studies. Thus, host specificity nodulation genes are located on the pSym megaplasmid.