SUMMARY: The F factor showed unstable maintenance in MP10. The properties of a more stable derivative () are described. was and carried the property and the ability to transfer the character. It contained a large plasmid of molecular weight about 129 x 10 daltons. The properties of and its derivatives suggested that the large plasmid was a cointegrate of F and the MP10 plasmid. transfer was efficient from to 799 MP10 and one exconjugant contained the intact cointegrate. The cointegrate was not successfully transferred to strains lacking MP10. It dissociated into apparently unaltered F and MP10 plasmids, but the deletion of small parts of one or both plasmids during cointegrate formation could not be ruled out. Cointegrate dissociation was more marked in 799 than in especially during growth in glucose–Casamino acids minimal medium. In the presence of R1, the cointegrate (like the MP10 plasmid) was stably maintained in the strain; maintenance of F was, however, unstable. It seems likely that replication of the cointegrate was controlled by the MP10 plasmid constituent.


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