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Chapter 17 : Conjugative Transposons: Transmissible Resistance Islands

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Abstract:

The features of conjugative transposons are similar enough to those of pathogenicity islands (PAIs) to raise the question whether some PAIs are actually conjugative transposons. This chapter reviews the features associated with conjugative transposons and other transmissible integrated elements. Conjugative transposons do not fit neatly into any of the traditional categories of mobile elements such as transposons, bacteriophages, and plasmids; instead, they combine features of all of them. The mechanisms of integration and excision are less well established for conjugative transposons. The types of lambdoid integrase genes found so far on most conjugative transposons are not easy to spot in BLAST searches and must be checked at the amino acid level for the conserved amino acid signature. The transfer regions of conjugative transposons appear to be stripped-down versions of the transfer regions of conjugative plasmids. The authors found the same thing in the conjugative transposons: the transfer region is less than 16 kbp, and there are few homologs to known plasmid transfer genes. Excision and transfer of at least some of the conjugative transposons are stimulated by the antibiotic tetracycline. The conjugative transposons can also act in to trigger excision and circularization of unlinked and apparently unrelated integrated elements called nonreplicating units (NBUs).

Citation: Salyers A, Shoemaker N, Bonheyo G, Frias J. 1999. Conjugative Transposons: Transmissible Resistance Islands, p 331-346. In Kaper J, Hacker J (ed), Pathogenicity Islands and Other Mobile Virulence Elements. ASM Press, Washington, DC. doi: 10.1128/9781555818173.ch17

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Circular Double-Stranded DNA
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Figures

Image of Figure 1
Figure 1

Steps in excision and transfer of a conjugative transposon. The conjugative transposon (shown as an open bar with differently hatched ends to indicate a lack of identity between the ends) is normally integrated into the chromosome. To transfer, it first excises to form a covalently closed circle, resealing the chromosomal site in the process, and then transfers by conjugation to a recipient. The single-stranded form of the donor and recipient circular forms is then copied (dashed line) to regenerate the double-stranded circular intermediate. The circular intermediate then integrates once again into the chromosome.

Citation: Salyers A, Shoemaker N, Bonheyo G, Frias J. 1999. Conjugative Transposons: Transmissible Resistance Islands, p 331-346. In Kaper J, Hacker J (ed), Pathogenicity Islands and Other Mobile Virulence Elements. ASM Press, Washington, DC. doi: 10.1128/9781555818173.ch17
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Image of Figure 2
Figure 2

Schematic representation of the structures of some of the better-studied gram-positive bacterial conjugative transposons. Areas with the same fill cross-hybridize with each other. The designations are the old names. Currently, we designate them CTn, CTn, CTn, CTn, CTn, and CTn5276 (top to bottom). CTn and CTn are nearly identical except for the extra antibiotic resistance genes (solid boxes) in CTn. CTn, ,and CTn cross-hybridize in the indicated regions but may not be as similar as CTn and CTn. Each of these three larger elements contains an insertion of a CTn-related element. CTn, from ,is completely unrelated to the others and carries a nisin gene and a gene for sucrose metabolism rather than the resistance genes seen on the other conjugative transposons.

Citation: Salyers A, Shoemaker N, Bonheyo G, Frias J. 1999. Conjugative Transposons: Transmissible Resistance Islands, p 331-346. In Kaper J, Hacker J (ed), Pathogenicity Islands and Other Mobile Virulence Elements. ASM Press, Washington, DC. doi: 10.1128/9781555818173.ch17
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Image of Figure 3
Figure 3

Schematic representation of the known conjugative transposons. The names in the figure are those under which information about the element was first published. Currently, we designate these elements CTn,CTn,CTn, CTn,and CTn, ( ) (top to bottom). CTn and CTn very similar to each other in the unfilled regions and are virtually identical in the area that contains ,and CTn has a 6- to 8-kbp insertion that contains Cln is a hybrid element with a CTn-type element embedded in another similarly sized element. CTn is a cryptic element that cross-hybridizes with CTn but is more distantly related to it than is CTn. CTn7S5i is unrelated to the other elements except that it contains ( ).

Citation: Salyers A, Shoemaker N, Bonheyo G, Frias J. 1999. Conjugative Transposons: Transmissible Resistance Islands, p 331-346. In Kaper J, Hacker J (ed), Pathogenicity Islands and Other Mobile Virulence Elements. ASM Press, Washington, DC. doi: 10.1128/9781555818173.ch17
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Image of Figure 4
Figure 4

Model for excision and integration of CTn ( ). The coupling sequences are marked with X(Y)s and Q(R)s to show that the coupling sequences do not base pair with each other. Thus, when the element excises, a 6-bp region of heterology is created that is resolved to copy one coupling sequence of the other ( ). Integration occurs by a similar process. Because of the mode of excision and integration, a conjugative transposon can bring along to its new site bases that were adjacent to one end in its former location (XXXXXX/YYYYYY in this case). This type of integration mechanism usually does not duplicate the target site, although it can appear to do so if a region near one end is identical to a region near the target site, as was the case with CTn ( ).

Citation: Salyers A, Shoemaker N, Bonheyo G, Frias J. 1999. Conjugative Transposons: Transmissible Resistance Islands, p 331-346. In Kaper J, Hacker J (ed), Pathogenicity Islands and Other Mobile Virulence Elements. ASM Press, Washington, DC. doi: 10.1128/9781555818173.ch17
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