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Chapter 10 : Conjugative Transposons and Related Mobile Elements

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Abstract:

This chapter covers the conjugative transposons from gram-positive bacteria and gram-negative bacteria, beginning with a detailed description of the enterococcal and streptococcal transposons Tn916 and Tn1545 and the larger streptococcal element Tn5253 and its components, Tn5251 and Tn5252. It also describes the conjugal transposons from and and spp. and mobilizable elements from and spp. The author describes the element of that encodes resistance to sulfamethoxazole and trimethoprim (SXT) that integrates into a unique target site in the genome of its host bacterium. He also detours into the world of integrating plasmids and the peculiar behavior of a class of elements originally described as conjugal plasmids assigned to the IncJ incompatibility group. Conjugative transposition of Tn916 is a relatively rare event, typically occurring at frequencies below 10 per donor bacterium. So far, only two factors have been reported to increase this frequency. One is addition of tetracycline, but increases in conjugative transposition frequency attributable to tetracycline are modest, and do not compare with the levels of induction of conjugative transposition of the conjugative transposons and related elements by tetracycline. The second is the nature of the coupling sequences flanking Tn916. The DNA sequences of the larger elements remain uncharted territory. Whereas some functions apart from antibiotic resistance that are encoded by the larger elements have been identified, the role these elements play in the biology of their bacterial hosts remains largely a mystery.

Citation: Churchward G. 2002. Conjugative Transposons and Related Mobile Elements, p 177-191. In Craig N, Craigie R, Gellert M, Lambowitz A (ed), Mobile DNA II. ASM Press, Washington, DC. doi: 10.1128/9781555817954.ch10

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Figures

Image of Figure 1.
Figure 1.

Genetic map of Tn. The thin gray line represents transposon DNA and the thick black arrows represent open reading frames. Four short reading frames are not shown: to the right of to the right of to the right of and to the right of . The position of the origin of conjugal DNA transfer, is indicated between and . The thin arrows indicate five promoters and their direction of transcription.

Citation: Churchward G. 2002. Conjugative Transposons and Related Mobile Elements, p 177-191. In Craig N, Craigie R, Gellert M, Lambowitz A (ed), Mobile DNA II. ASM Press, Washington, DC. doi: 10.1128/9781555817954.ch10
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Image of Figure 2.
Figure 2.

Binding sites for Int and Xis at the ends of Tn. The thick black line represents DNA from each end of the transposon. The diamonds labeled Int-C show where the C-terminal domains of Int bind to the transposon ends and flanking bacterial DNA. The black triangles labeled DR-2 and Int-N show the positions and relative orientation of binding sites for the N-terminal domain of integrase. The open triangles labeled Xis show the positions and relative orientation of binding sites for Xis.

Citation: Churchward G. 2002. Conjugative Transposons and Related Mobile Elements, p 177-191. In Craig N, Craigie R, Gellert M, Lambowitz A (ed), Mobile DNA II. ASM Press, Washington, DC. doi: 10.1128/9781555817954.ch10
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Image of Figure 3.
Figure 3.

Model for Tn transposition. The thick lines represent Tn and the thin lines represent the DNA adjacent to the transposon. Coupling sequences are indicated by the hypothetical nucleotide pairs X-Y, Q-R, and A-B. (A) Cleavage of one DNA strand at each end of the transposon on the 5′ side of the coupling sequence, indicated by the vertical arrows, followed by DNA strand exchange leads to the formation of a Holliday junction intermediate. A second round of cleavage and DNA strand exchange results in the formation of an excised circular intermediate form of the transposon that contains a heteroduplex region formed from the base pairs originally present in the coupling sequences flanking the transposon in the donor. The reciprocal product can be processed by DNA replication to yield a pair of excisant molecules, each carrying one of the coupling sequences originally flanking the transposon. (B) After introduction of a single strand of the circular intermediate form of the transposon into the recipient, the complementary strand is synthesized to form a new intermediate with only one of the coupling sequences originally flanking the transposon in the donor. A recombination event similar to that shown in panel A results in the transposon being integrated into the recipient DNA where it is flanked on each side by a heteroduplex region composed of coupling sequence and target DNA. After replication, two DNA molecules are produced with sequences from the circular form of the transposon at either the left or the right of the integrated transposon.

Citation: Churchward G. 2002. Conjugative Transposons and Related Mobile Elements, p 177-191. In Craig N, Craigie R, Gellert M, Lambowitz A (ed), Mobile DNA II. ASM Press, Washington, DC. doi: 10.1128/9781555817954.ch10
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