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Chapter 8 : Conjugative and Mobilizable Transposons

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Abstract:

Conjugative transposons—integrated elements that excise themselves from the chromosome of the donor bacterium, transfer themselves by conjugation to another bacterium, and integrate into the chromosome of the recipient cell—are the focus of this chapter. The chapter also briefly covers smaller integrated elements called mobilizable transposons. Mobilizable transposons are triggered to excise by conjugative transposons, which also mobilize them to a recipient cell. The study of such elements is revealing novel mechanisms of excision, integration, and transfer. The lack of a large database of information on conjugative and mobilizable transposons has led to two problems. One is the lack of a full range of "sequence signatures" that would reveal the presence of a conjugative transposon in a genome sequence. Second, it is now clear that there is a considerable amount of diversity in this group of elements, diversity that has led to some uncertainty as to how to define the category "conjugative transposon". Mobilizable transposons (MTns) are smaller integrated elements that transfer with the assistance of a CTn. To date, the most extensively studied conjugative transposons are the conjugative transposon, CTnDOT, and the gram-positive conjugative transposon, Tn916. Excision and transfer of CTnDOT and Tn916 are regulated by tetracycline, but the mechanisms of regulation are different. CTn-like integrated, self-transmissible elements have been found in many different types of bacteria. MTns, like CTns, have a circular intermediate but rely on a CTn for transfer functions. A summary of the features of the integrated elements is covered in the chapter.

Citation: Salyers A, Whittle G, Shoemaker N. 2004. Conjugative and Mobilizable Transposons, p 125-143. In Miller R, Day M (ed), Microbial Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555817749.ch8

Key Concept Ranking

Integrative and Conjugative Elements
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Gene Expression and Regulation
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Figures

Image of FIGURE 1
FIGURE 1

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 , with permission.)

Citation: Salyers A, Whittle G, Shoemaker N. 2004. Conjugative and Mobilizable Transposons, p 125-143. In Miller R, Day M (ed), Microbial Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555817749.ch8
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Image of FIGURE 2
FIGURE 2

Model proposed for the excision and integration of Tn 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 Tn. This model is based on the model described by .

Citation: Salyers A, Whittle G, Shoemaker N. 2004. Conjugative and Mobilizable Transposons, p 125-143. In Miller R, Day M (ed), Microbial Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555817749.ch8
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Image of FIGURE 3
FIGURE 3

(A) Location of genes on CTnDOT that are essential for integration (int), excision (, , and ), regulation (), and transfer (). The region is a 13-kbp composite of transposon and mobilizable transposon genes ( ). (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 (step 2). RteC activates transcription of genes in the excision region (step 3) and at least some genes (step 4). Steps 3 and 4 appear to occur simultaneously. + indicates that induction occurred.

Citation: Salyers A, Whittle G, Shoemaker N. 2004. Conjugative and Mobilizable Transposons, p 125-143. In Miller R, Day M (ed), Microbial Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555817749.ch8
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Image of FIGURE 4
FIGURE 4

(A) The location of genes on Tn 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 Tn excision and transfer genes are regulated (adapted from ). Exposure to tetracycline increases the number of transcripts that pass through , , and . In the circular form of the excised element, these transcripts would cross the joined ends and move through the transfer region (see panel A).

Citation: Salyers A, Whittle G, Shoemaker N. 2004. Conjugative and Mobilizable Transposons, p 125-143. In Miller R, Day M (ed), Microbial Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555817749.ch8
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Image of FIGURE 5
FIGURE 5

Some CTns mobilize elements in . 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 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.

Citation: Salyers A, Whittle G, Shoemaker N. 2004. Conjugative and Mobilizable Transposons, p 125-143. In Miller R, Day M (ed), Microbial Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555817749.ch8
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References

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Tables

Generic image for table
TABLE 1

Examples of conjugative and mobilizable transposon families

Citation: Salyers A, Whittle G, Shoemaker N. 2004. Conjugative and Mobilizable Transposons, p 125-143. In Miller R, Day M (ed), Microbial Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555817749.ch8

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