SUMMARY: A His Nif K12, Hfr strain (UNF43) was constructed by an intergeneric mating between a donor strain (HF3) and a His, Hfr strain (SBI824) which transfers as an early marker. An F-prime plasmid, FN39, carrying genes which correspond to the chromosomal region, , but excluding and , was isolated from UNF43. Translocation of carbenicillin resistance genes from a P-type R-factor, R68, to FN39 increased the stability of and on the derivative F-prime, FN68.

Sedimentation analysis of both F-primes in sucrose gradients revealed four covalently closed circular(CCC) DNA species of molecular weights 279 + 9,136 + 3,90 + 1 and 44 + 1 megadaltons. It is suggested that the two smallest CCC-DNA species are component replicons of the composite F-primes of molecular weight 136 + 3 megadaltons, and that the molecules of 279 + 9 megadaltons are CCC-dimers.

FN68 was transferable in intergeneric matings to and but not to ; only carbenicillin resistance and sex factor activity were transferred to genes on FN68 were expressed in a Nif mutant of and also in , which in conventional tests is naturally non-nitrogen-fixing; expression of the determinant of FN68 became temperature-sensitive in


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