Summary: The genetic diversity of symbiosis (Sym) plasmids was investigated in samples from two field populations of biovar that had previously been characterized for chromosomally-encoded enzyme electrophoretic polymorphism. Five overlapping cloned DNA fragments from the Sym plasmid pRL1JI were used as hybridization probes to identify restriction fragment variation in the homologous genes of the isolates. In addition, a clone of the β-galactosidase gene region was used as a probe, extending the data on chromosomal relatedness. The plasmid-encoded Sym region was very polymorphic (11.4% average DNA sequence divergence). Some isolates had the same Sym fragment pattern as pRL1JI, but most were very different. One was closely similar to pRL6JI, another widely-studied plasmid. The distribution of plasmids across chromosomal backgrounds was far from random, as though the species were subdivided into compartments with largely separate plasmid pools. Nevertheless, indistinguishable plasmids were found in quite different genetic backgrounds, implying that plasmid transfer must occur in the field. This has implications for the population genetics and evolution of bacteria and for the release of genetically engineered strains.


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