Summary: Plasmids obtained from four field isolates of were visualized following electrophoresis on agarose gels. The relative mobilities of the bands observed corresponded to plasmids with molecular weights of about 100 × 10 and greater. Each field isolate examined had a different pattern of plasmids.

Lysates from strain 300 normally produced three plasmid bands, although in some preparations two much larger plasmids were also visible. The smallest plasmid band seen in strain 300 probably contains two co-migrating plasmids, because in one derivative of strain 300 it was replaced by a doublet, presumably as a result of the presence of a small deletion in one of the co-migrating plasmids. No apparent symbiotic defects were associated with the presence of this deletion. However, a non-nodulating derivative of strain 300 (strain 6015) was found to have suffered a deletion in the third-smallest plasmid of this strain.

Transfer of the determinant of -sized bacteriocin production pRL1JI (from isolate 248) was correlated with the appearance of an extra plasmid with a molecular weight of about 130 × 10. Another determinant of bacteriocinogenicity, pRL4JI (from isolate 309), was correlated with the presence of an extra plasmid of the same size (about 160 × 10) as one seen in the donor strain 309. The third determinant, pRL3JI (from isolate 306), could not be correlated with the presence of an extra plasmid of the same size as any in strain 306, and it appears that recombination occurred between pRL3JI and plasmids resident in strain 300.


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