Figure 1.

Conjugative plasmids (CPs) and integrative conjugative elements (ICEs). Events leading to horizontal transfer of CPs (A) or ICEs (B) are shown schematically. Before transfer can occur, tra genes must be expressed and a T4SS assembled. After cell-cell contact formation, transfer competent donor cells initiate a rolling circle type replication from circular dsDNA and translocate ssDNA via the T4S machinery into recipient cells. dsDNA is then reconstituted in the recipient (dotted inner circle). (A) CPs can autonomously replicate due to the presence of rep genes. (B) ICEs replicate as integrated elements with the host chromosome (green lines), integration and excision is mediated by int/xis genes required for integration and excision by site-specific recombination via attachment sites (vertical bars). After excision and before integration, ICEs are present in a plasmid-like dsDNA form.