Circular linkage map of Rhizobium meliloti chromosome

Abstract

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 transformation^1–3 and transduction⁴ 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 species^6,7. Some of these plasmids can promote chromosome transfer in Pseudomonas aeruginosa strain PAT but show little chromosome donor ability (Cda) in strain PAO^8,9. 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.