DISTRIBUTION OF REPC PLASMID-REPLICATION SEQUENCES AMONG PLASMIDS ANDISOLATES OF RHIZOBIUM-LEGUMINOSARUM BV VICIAE FROM FIELD POPULATIONS

The distribution of four classes of related plasmid replication genes (repC) within three field populations of Rhizobium leguminosarum in Fr ance, Germany and the UK was investigated using RFLP, PCR-RFLP and pla smid profile analysis. The results suggest that the four repC classes are compatible: when two or more different repC sequences are present in a strain they are usually associated with different plasmids. Furth ermore, classical incompatibility studies in which a Tn5-labelled plas mid with a group IV repC sequence was transferred into field isolates by conjugation demonstrated that group IV sequences are incompatible w ith each other, but compatible with the other repC groups. This suppor ts the idea that the different repC groups represent different incompa tibility groups. The same field isolates were also screened for chromo somal (plac12) and symbiotic gene (nodD-F region) variation. Compariso n of these and the plasmid data suggest that plasmid transfer does occ ur within field populations of R. leguminosarum but that certain plasm id-chromosome combinations are favoured.