CONJUGATIVE TRANSPOSONS - AN UNUSUAL AND DIVERSE SET OF INTEGRATED GENE-TRANSFER ELEMENTS

Conjugative transposons are integrated DNA elements that excise themse lves to form a covalently closed circular intermediate. This circular intermediate can either reintegrate in the same cell (intracellular tr ansposition) or transfer by conjugation to a recipient and integrate i nto the recipient's genome (intercellular transposition). Conjugative transposons were first found in gram-positive cocci but are now known to be present in a variety of gram-positive and gram-negative bacteria also. Conjugative transposons have a surprisingly broad host range, a nd they probably contribute as much as plasmids to the spread of antib iotic resistance genes in some genera of disease-causing bacteria. Res istance genes need not be carried on the conjugative transposon to be transferred. Many conjugative transposons can mobilize coresident plas mids, and the Bacteroides conjugative transposons can even excise and mobilize unlinked integrated elements. The Bacteroides conjugative tra nsposons are also unusual in that their transfer activities are regula ted by teracycline via a complex regulatory network.