Symbiotic Plasmid Rearrangement in Rhizobium leguminosarum bv. viciae VF39SM

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