Symbiotic plasmid rearrangement in Rhizobium leguminosarum bv. viciae VF39SM

A rearrangement between the symbiotic plasmid (pRleVF39d) and a nonsymbioti c plasmid (pRleVF39b) in Rhizobium leguminosarum by. viciae VF39 was observ ed. The rearranged derivative showed the same plasmid profile as its parent strain, but hybridization to nod, fix, and nif genes indicated that most o f the symbiotic genes were now present on a plasmid corresponding in size t o pRleVF39b instead of pRleVF39d. On the other hand, some DNA fragments ori ginating from pRleVF39b now hybridized to the plasmid band at the position of pRleVF39d, These results suggest that a reciprocal but unequal DNA excha nge between the two plasmids had occurred.