Structural organization of a 67-kilobase streptococcal conjugative element mediating multiple antibiotic resistance.

The molecular organization of the conjugative cat-erm-tet region of Streptococcus agalactiae B109 was examined by cloning large contiguous portions of the strain B109 chromosome, using a cosmid vector system. The organization of this region was compared with pDP5, a plasmid which acquired this resistance element by transposition. Both the chromosomal copy and the transposed copy of the resistance region were found to be 67-kilobases long, although sequences at the boundary of the transposed copy of the element showed some rearrangement. In addition to the stable chromosomal state, we present evidence which suggests the presence of a circular form of the element.