When the Challis strain of was transformed by the 17 megadalton β plasmid from strain DS 5, the plasmid underwent a 1.5 megadalton deletion (LeBlanc & Hassell, 1976). Furthermore, the covalently closed circular (CCC) plasmid DNA isolated from Challis transformants was rapidly converted to a linear form which did not possess any detectable transforming activity. To obtain stable CCC plasmid DNA a competent culture of a Lancefield group F streptococcus, strain DL 8 (ATCC 12393), was used as a recipient of β plasmid DNA. The plasmid DNA isolated from group F transformants exhibited the same configuration and size characteristics as the DS 5 β plasmid, and the CCC configuration was stable upon storage. CCC plasmid DNA from a group F transformant was biologically active and, when added to competent cultures of strain DL 8, transformed them at frequencies about 100-fold greater than did β plasmid DNA from DS 5. This suggests the existence of a restriction-modification system in strain DL 8.


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