A new symbiotic cluster on the pSym megaplasmid of Rhizobium meliloti 2011 carries a functional fix gene repeat and a nod locus.

A 290-kilobase (kb) region of the Rhizobium meliloti 2011 pSym megaplasmid, which contains nodulation genes (nod) as well as genes involved in nitrogen fixation (nif and fix), was shown to carry at least six sequences repeated elsewhere in the genome. One of these reiterated sequences, about 5 kb in size, had previously been identified as part of a cluster of fix genes located 220 kb downstream of the nifHDK promoter. Deletion of the reiterated part of this fix cluster does not alter the symbiotic phenotype. Deletion of the second copy of this reiterated sequence, which maps on pSym 40 kb upstream of the nifHDK promoter, also has no effect. Deletion of both of these copies however leads to a Fix- phenotype, indicating that both sequences carry functionally reiterated fix gene(s). The fix copy 40 kb upstream of nifHDK is part of a symbiotic cluster which also carries a nod locus, the deletion of which produces a marked delay in nodulation.