Chapter 8 : Conjugative and Mobilizable Transposons

Steps in the transfer of a conjugative transposon. The integrated element, represented by the thick bar, excises to form a circular intermediate. The ends are highlighted to show where they are located in the circular form and to emphasize the fact that the transferred element integrates via its ends. A single-stranded copy of the circular form is transferred through a multiprotein mating apparatus that joins the donor and recipient cells. The single-stranded copy becomes double stranded before integrating into the recipient chromosome. (Adapted from Whittle et al., 2002 , with permission.)

Model proposed for the excision and integration of Tn916 and CTnDOT. The coupling sequences, which are chromosomal sequences, are represented by XXX/YYY or QQQ/RRR to indicate that initially they are complementary in sequence. XXX does not base pair with RRR and YYY does not base pair with QQQ. Vertical black arrows indicate the location of the staggered cuts that are produced prior to excision or integration. In the illustration of CTnDOT excision and integration, a thick bar (consensus sequence GTAnnTTTGC) identifies a sequence that is found at one end of CTnDOT immediately adjacent to the coupling sequence and in the site into which CTnDOT integrates. This sequence may account, at least in part, for the fact that CTnDOT integration is less random than integration of Tn916. This model is based on the model described by Scott and Churchward (1995) .

(A) Location of genes on CTnDOT that are essential for integration (int), excision (orf2c, orf2d, and exc), regulation (rte), and transfer (tra). The ermF region is a 13-kbp composite of transposon and mobilizable transposon genes ( Whittle et al., 2001 ). (B) An expanded view of the excision and regulatory regions of CTnDOT in which the main layers of regulation are illustrated by arrows. Exposure to tetracycline (step 1) leads to increased production of TetQ, KteA, and RteD. RteB, in turn, activates transcription of rteC (step 2). RteC activates transcription of genes in the excision region (step 3) and at least some tra genes (step 4). Steps 3 and 4 appear to occur simultaneously. + indicates that induction occurred.

(A) The location of genes on Tn916 that are important for integration, transfer, and excision. Most of these genes are transcribed in the same direction, as indicated by the arrows. (B) A simplified version of how Tn916 excision and transfer genes are regulated (adapted from Celli and Trieu-Cuot, 1998 ). Exposure to tetracycline increases the number of transcripts that pass through tetM, xis, and int. In the circular form of the excised element, these transcripts would cross the joined ends and move through the transfer region (see panel A).

Some CTns mobilize elements in trans. The mobilizable coresident plasmid provides the pro-tein(s) that nick the plasmid at the transfer origin. The CTn provides the multiprotein mating apparatus through which the single-stranded copy of the plasmid is mobilized (mob) into the recipient cell. Some CTns, such as CTnDOT, also act in trans to trigger excision and mobilization of coresident mobilizable transposons (MTns). In this case, the CTn provides factors that stimulate excision and circularization of the MTn and the transfer (tra) functions that allow the single-stranded copy of the MTn to be transferred to the recipient cell. The MTns integrate on their own without help from the CTn.