An excision reporter plasmid was constructed to characterize the intracellular mobility of Tn in various Gram-positive bacteria. The reporter component of this plasmid is a chloramphenicol-resistance gene which has been insertionally inactivated with the integrative vector pAT112 containing the attachment site of Tn Tn-mediated excision of pAT112, to produce clones resistant to chloramphenicol, was detected in BM4110, L028-Str and BM120, but not in MG1363-RF or in BM124, and always depended upon the ability of the bacterial host to generate circular forms of the transposon. The results suggest that (i) the excision event, although required, is not sufficient for conjugal transfer to occur and (ii) there is no linear relationship between the donor potential of strains and either the excision frequency of pAT112 or the copy number of Tn circular intermediates per cell in these hosts. Excision of pAT112 occurred mainly during the late exponential phase of growth of and monocytogenes and this recombination event was not stimulated by heat shock, salt and alcohol stresses or by the presence of tetracycline in the medium.


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