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Category: Bacterial Pathogenesis
Conjugative Transposons, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815615/9781555813031_Chap17-1.gif /docserver/preview/fulltext/10.1128/9781555815615/9781555813031_Chap17-2.gifAbstract:
Transfer of conjugative transposons requires cell-to-cell contact and occurs at relatively low frequency. However, the host range of the various elements is, in some cases, quite diverse. In this chapter, the author reviews the current state of knowledge of the epidemiology, resistance profiles, and transposition mechanisms of the major types of characterized conjugative transposons found in human pathogenic bacteria. A recently published survey directly implicates conjugative transposons in the steady and significant rise in erythromycin and tetracycline resistance in Bacteroides species over the past three decades. Unlike Tn916 family transposons, Bacteroides conjugative transposons can mobilize coresident nonconjugative transposons. Bacteroides conjugative transposons can be transferred in vitro into distantly related species, such as Escherichia coli. The Tn916 family elements are the most thoroughly studied and characterized of the conjugative transposons. The single "family" of Bacteroides conjugative transposons described to date is the CTnDOT family. This family is characterized by the presence of right and left ends that differ from each other but are conserved within the family. The rteA, rteB, and rteC genes are also involved in excision, presumably through a regulatory role. rteA and rteB are required for expression of rteC. Expression of rteC increases transcription of exc, thereby increasing excision of CTnDOT. Recent advances in understanding the structural details of λ and related integrase interactions with DNA targets and cooperative proteins have the potential to stimulate intelligent, structure-based design of such chemical inhibitors.
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Graphic depiction of 65-kb composite transposon Tn5385 from E. faecalis. Tn5385 is a conjugative element that transfers between enterococci at low frequency and integrates in most cases into the recipient chromosome in a site-specific manner. Within Tn5385 lies a typical Tn 916- like conjugative transposon (Tn5381) that can transfer by itself or with the larger element. When transferring by itself, Tn5381 is relatively nonselective in its insertion sites. Regions of different lineages are connected within Tn5385 by different insertion elements (see the text). Reprinted with permission from reference 43 .
(Top) Map of Tn916. Identity of the specific ORFs and their directions of transcription are identified below the map, as are the functions of the regions, where that is known. The origin of transfer is indicated (oriT) at the right-hand end of the transposon. Reprinted with permission from reference 30 . (Bottom) Graphic depiction of expression of ORFs in the left end of Tn916 with or without exposure to tetracycline. Black arrows represent genes that are expressed under the identified conditions. In the absence of tetracycline, expression of orf9 negatively regulates transcription of orf7 and orf8. Transcription from the tet promoter (upstream of orf12) is also terminated within orf12. In the presence of tetracycline, transcription from the tet promoter moves through the palindromes within orf12 and into tet (M). Transcription then continues through tet (M), negatively regulating expression of the gene products of orf9, which ultimately leads to transcription of orf7 and orf8, which positively regulate their own transcription. This results in increased expression of int and xis, stimulating excision and circularization. Once circularized, transcription can continue through the newly formed joint and into the transfer genes found in the right-hand end of the transposon. Adapted from data published in reference 11 .
(Top) Comparison of structures of Bacteroides conjugative transposons CTnDOT and CTnERL. The transposons are identical except for the insertion of the ermF region in CTnDOT. The individual ORFs that make up the ermF region of CTnDOT are detailed. Reprinted with permission from reference 63 . (Bottom) Structure of CTnGERM1, a non-CTnDOT-family Bacteroides conjugative transposon. Reprinted with permission from reference 61.
Similar regions found within CTnR391 conjugation region (identical to corresponding region of SXT) and other transferable elements from gram-negative bacteria. Reprinted with permission from reference 6 .
Comparison of clostridial conjugative transposons Tn5397 and Tn916. Differences include the ends (where orf5, orf25, xis, and int) are missing from Tn5397. The ends of the transposons also differ. Tn5397 also contains a group II intron within orf14, a serine recombinase (tndX) gene in place of int and xis, and two additional ORFs (orf25 and orf26) upstream of the tet (M) gene. Reprinted with permission from reference 47 .
Conjugative transposons discussed in this chapter