Excision and transfer of bacteroides conjugative integrated elements

Integrated conjugative elements (ICEs), previously called conjugative transposons (CTns), are proving to be as widespread in bacteria as conjugative piasmids, and they are making a significant contribution to bacterial evolution. CTns carry a variety of accessory genes, such as antibiotic resistance and virulence genes, but even the ones that do not carry such genes (cryptic CTns) are poised to acquire and spread accessory genes in the future. The mechanisms of excision and integration of these mobile elements have only recendy begun to be understood. The most extensively studied CTn excision and integration system is the one used by a family of Bacteroides CTns related to CTnDOT. Members of this family of CTns are clearly very successful mobile elements since they are now found in over 70% of natural Bacteroides isolates, an increase from 20-30% before 1970. Information about the complex excision system CTnDOT has not only provided information about why CTnDOT type elements are so successful at being spread and maintained in the colonic ecosystem, but has also expanded the paradigm of excision that is based on earlier studies of the lysogenic phages and the Gram-positive CTn, Tn916. The regulatory cascade that regulates excision and transfer genes in response to stimulation of donor cells by tetracycline is complex and is unusual in that some of the proteins that regulate expression of transfer genes play a structural role in excision. This complex regulatory system may help to explain why CTns in this family are so stably maintained in colonic Bacteroides and related genera, even in the absence of selection.