PRESENCE OF UNIQUE REPEATED INSERTION SEQUENCES IN NODULATION GENES OF RHIZOBIUM-HEDYSARI

Cloning and sequencing of DNA from a symbiotic large plasmid in Rhizob ium 'hedysari' strain IS 123 required for its nodulation of the medite rranean legume crop Hedysarum coronarium (sulla) and complementation s tudies of nod(-) mutant derivatives led to the characterization of a 3 0-kb region containing common and host-specific nod genes. This DNA re gion also contained at least six copies of a novel insertion sequence- like structure, same of which appeared to have suffered deletions. Thi s 0.8 kb novel element carries two 17-bp flanking inverted repeats and an open reading frame showing homology with a transposase from Staphy lococcus aureus. Hybridization studies revealed that several strains o f Rhizobium 'hedysari' carry this element in various copy number. The six copies in strain IS 123 appear clustered specifically within the p Sym nod region. The significance of this IS element in rhizobia and it s possible use as a probe for taxonomic and phylogenetic studies of Rh izobiaceae is addressed.