SUMMARY: Plasmids pRL 1JI and pRL6JI, which carry determinants essential for symbiosis in field isolates 248 and 128C53, respectively, were both incompatible with two other transmissible plasmids that did not carry symbiotic determinants. When derivatives of these two plasmids (pRL3JI or pRL4JI) were introduced into a strain which already contained pRL 1JI or pRL6JI, recombinant replicons were often obtained: these were of uniform size for each pair of incompatible plasmids. Recombinant nodulation plasmids were also observed following infrequent interactions with pRL10JI, a natural nodulation plasmid that does not belong to the same incompatibility group as pRL3JI or pRL4JI.

By the formation of a recombinant nodulation plasmid, it has been possible to transfer the determinants for nodulation ability (Nod), nitrogen fixation (Fix) and hydrogen uptake (Hup) from pRL6JI, a plasmid that was not self-transmissible, on to a replicon that was transmissible at high frequency and carried a selectable drug resistance marker. In the case of pRL10JI, which is also a non-transmissible nodulation plasmid, the formation of a recombinant nodulation plasmid transferred the symbiotic determinants to a transmissible replicon belonging to a different incompatibility group from pRL10JI itself.


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