Cyclic AMP regulation of the mob operon of the non-self-transmissible plasmid pEC3 isolated from a phytopathogenic bacterium

A non-self-transmissible multiple-copy plasmid, pEC3, isolated from the phy topathogenic bacterium, Erwinia carotovora subsp. carotovora, can be mobili zed with the help of a self-transmissible plasmid. When donors carrying a c ya or a crp mutation were used, the mobilization efficiencies of pEC3 were markedly decreased. The covalently closed circular conformation of plasmid DNA containing both oriT and mob genes of pEC3 was found to be relaxed afte r the addition of cyclic AMP (cAMP). The relaxed DNA contained a specific s ingle-strand DNA nick within the oriT region. RNA transcription of the mob operon was also increased by the addition of cAMP. These results indicate t hat the expression of the mob operon of pEC3 is positively regulated by cAM P and that the mob gene products are involved in nicking within the oriT re gion.