Transposon-like structure of a new plasmid-encoded restriction-modi®cation system in Rhizobium leguminosarum VF39SM

Total DNA isolated from Rhizobium leguminosarum VF39SM cells is resistant to cleavage by the restriction endonuclease PstI. Plasmid curing and transfer studies localized this phenotype to pRleVF39b, the second smallest of six plasmids found in this bacterium. In vitro selection for vector modi®cation was employed to isolate a presumptive methylase gene (M.Rle39BI) from a plasmid gene library. Total and plasmid DNAs isolated from E. coli containing M.RleBI were resistant to digestion by PstI. Sequence data suggested that a putative restriction endonuclease (R.Rle39BI) was also encoded on the same fragment. The two genes were ̄anked by identical copies of a putative insertion sequence, which was also present in several copies elsewhere in the VF39SM genome. The presence of this element in other strains examined suggested that this element is indeed an insertion sequence. The di€erences in G/C content between the DNA coding for the R/M system and that of the IS element suggest that this DNA region may have been acquired by horizontal transfer.