Distribution of repC plasmid-replication sequences among plasmids and isolates of Rhizobium leguminosarum bv. viciae from field populations.

The distribution of four classes of related plasmid replication genes () within three field populations of in France, Germany and the UK was investigated using RFLP, PCR-RFLP and plasmid profile analysis. The results suggest that the four classes are compatible: when two or more different sequences are present in a strain they are usually associated with different plasmids. Furthermore, classical incompatibility studies in which a Tn5-labelled plasmid with a group IV sequence was transferred into field isolates by conjugation demonstrated that group IV sequences are incompatible with each other, but compatible with the other groups. This supports the idea that the different groups represent different incompatibility groups. The same field isolates were also screened for chromosomal (12) and symbiotic gene ( region) variation. Comparison of these and the plasmid data suggest that plasmid transfer does occur within field populations of but that certain plasmid-chromosome combinations are favoured.