Identification of a Rhizobium meliloti pSym2011 region controlling the host specificity of root hair curling and nodulation.

In Rhizobium meliloti 2011 nodulation genes (nod) required to nodulate specifically alfalfa are located on a pSym megaplasmid. Nod- derivatives carrying large pSym deletions were isolated. By complementation of these strains with in vivo- and in vitro-constructed episomes containing pSym of sequences and introduction of these episomes into Agrobacterium tumefaciens, we show (i) that from a region of pSym of about 360 kilobases, genes required for specific alfalfa nodulation are clustered in a DNA fragment of less than 30 kilobases and (ii) that a nod region located between nifHDK and the common nod genes is absolutely required for alfalfa nodulation and controls the specificity of root hair curling and nodule organogenesis initiation.