Cloning and physical characterization of chromosomal conjugative elements in streptococci

We used a directed insertion method to introduce a nonreplicating vector plasmid into the large conjugative cat-tet element found in the chromosome of Streptococcus pneumoniae BM6001 and derivatives. To direct insertion preferentially to the conjugative element, we transferred it by conjugation to Streptococcus faecalis and then used DNA from this strain as a source of restriction nuclease fragments for ligation to digests of the vector pVA891, which can replicate in Escherichia coli but not in streptococci. This ligation mix was used to transform pneumococcal cells carrying the cat-tet element, with selection for the erythromycin resistance carried by pVA891. Eight such isolates were found, and transformation and conjugation tests showed that in each case the vector had inserted into the conjugative element, as expected. DNA from these pneumococcal strains generated a variety of E. coli plasmids which provide tools for obtaining a detailed restriction map and for defining other structural features of the streptococcal conjugative element.