GENETIC studies on symbiotic nitrogen-fixing rhizobia are greatly hampered by the lack of an efficient system for genetic mapping in rhizobia. Although there have been reports on transformation1-3 and transduction 4 in nodulating rhizobia as well as conjugation5 in a non-nodulating strain of Rhizobium lupini, the genetic map of a nodulating and nitrogen-fixing Rhizobium strain has not been established yet. Plasmids of the P-1 incompatibility group have a remarkably wide host range and can be transferred to many Rhizobium species6,7. Some of these plasmids can promote chromosome transfer in Pseudomonas aeruginosa strain PAT but show little chromosome donor ability (Cda) in strain PAO8,9. Recently a variant (R68.45) of the P group plasmid R68, with high Cda property in strain PAO, was isolated by Haas and Holloway10. Plasmid R68.45, unlike most of the sex factors, promotes chromosome transfer for a range of chromosomal genes in Rhodopseudomonas, P. putida, Escherichia coil (B. W. Holloway, personal communication) and in Rhizobium leguminosarum11. We have found that this plasmid can promote chromosome transfer also in Rhizobium meliloti and can be used for mapping of the chromosome. We present here a circular genetic map of the R. meliloti 41 chromosome and describe a new approach in the evaluation of conjugational mapping data which may be applicable to other Gram-negative bacteria.
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