USE OF AN EXCISION REPORTER PLASMID TO STUDY THE INTRACELLULAR MOBILITY OF THE CONJUGATIVE TRANSPOSON TN916 IN GRAM-POSITIVE BACTERIA

An excision reporter plasmid was constructed to characterize the intra cellular mobility of Tn916 in various Cram-positive bacteria. The repo rter component of this plasmid is a chloramphenicol-resistance gene wh ich has been insertionally inactivated with the integrative vector pAT 112 containing the attachment site of Tn916. Tn916-mediated excision o f pAT112, to produce clones resistant to chloramphenicol, was detected in Enterococcus faecalis BM4110, Listeria monocytogenes L028-Str and Streptococcus gordonii BM120, but not in Lactococcus lactis MG1363-RF or in Streptococcus pneumoniae BM124, and always depended upon the abi lity of the bacterial host to generate circular forms of the transposo n. 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 E. faecalis strain s and either the excision frequency of pAT112 or the copy number of Tn 916 circular intermediates per cell in these hosts. Excision of pAT112 occurred mainly during the late exponential phase of growth of E. fae calis and L. monocytogenes and this recombination event was not stimul ated by heat shock, salt and alcohol stresses or by the presence of te tracycline in the medium.