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Timestamp: 2019-04-26 10:11:29+00:00

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Tn7transposition is modulated by different aspects of target DNAs. Tn7is preferentially attracted to attTn7,a single specific site in the chromosomes of many bacteria, a lifestyle that promotes vertical transmission of the element. Tn7is also preferentially attracted to conjugating plasmids, likely because of the unique form of lagging-strand synthesis that occurs upon mating. These different targeting pathways are mediated by different subsets of Tn7-encoded transposition proteins. Preferential insertion on conjugating plasmids provides a lifestyle that promotes dispersal of Tn7between organisms. Tn7avoids DNA containing Tn7,thereby avoiding self-destruction.
Tn7translocates via cut-and-paste transposition. The substrate DNAs are a donor DNA containing Tn7and a target plasmid containing attTn7.Recombination initiates by double-strand breaks at either end of the Tn7element; a second break generates an excised transposon species that is then inserted into attTn7.The other product of the reaction is a gapped donor backbone plasmid.
The ends of Tn7.The Tn7L and Tn7R end segments contain multiple copies of the 22-bp TnsB binding sequence as defined by TnsB-footprinting studies. The 30-bp nearly perfect inverted repeats contain a perfect 8-bp terminal inverted repeat (bold) containing a critical 5′-TGT . . . .ACA-3′, a 5-bp spacer region and a TnsB binding site.
Tn7insertion into attTn7.The specific point of Tn7insertion lies between pstSand glmS.Tn7inserts into attTn7in a specific orientation with the right end of Tn7adjacent to glmS.The position designated 0 is the center of the target 5 bp duplicated upon Tn7insertion, base pairs rightward toward oriCare designated  and base pairs leftward are designated –.
Tn7transposition chemistry. In the donor site, Tn7is flanked by a 5-bp duplication of target sequences resulting from the insertion of Tn7into that site. Tn7is excised from the donor site by double-strand breaks. Cleavage at the transposon ends is staggered: cleavage occurs precisely at the 3′ ends of the element but occurs within the flanking donor DNA such that 3 nucleotides of flanking donor sequences (indicated by “d”) are attached to the 5′ ends of the transposon. The 3′ ends of the transposon attack staggered positions on the target DNA (indicated by “t”), generating a new insertion flanked by short gaps. The donor nucleotides on the 5′ ends of the transposon are removed and the gaps are repaired by host functions.
The processing events at the ends of Tn7are mediated by two different polypeptides. TnsB mediates the cleavages at the 3′ ends of Tn7and the joining of the exposed 3′-OHs to the target DNA. TnsA mediates the cleavages at the 5′ ends of Tn7.Transposase activity requires TnsA and TnsB: no activity is seen in the presence of either protein alone.
Double-strand breaks at transposon ends can occur by multiple mechanisms. Whereas Tn7uses cleavages by two different polypeptides to generate double-strand breaks at the ends of Tn7,IS10and IS50use a one polypeptide and a hairpinning mechanism to generate double-strand breaks. With IS10and IS50,a nick is introduced at the 3′ end of the transposon. The resulting 3′-OH intramolecularly attacks the 5′ strand of the transposon to form a hairpin on the transposon end; this step also disconnects the transposon from the flanking donor DNA. The transposase then opens the hairpin, again exposing the 3′ end of the transposon which can go on to attack the target DNA.
The gapped donor site can be repaired. The gapped donor site that results from transposon excision can be repaired by double-strand break repair using another copy of the donor site. Such repair can be visualized by examination of cells containing lac – heteroalleles: one lac::Tn7allele and the other a lac – heteroallele mutant at a site that does not overlap the Tn7insertion site. When Tn7transposition occurs, that is, Tn7excises from the lac::Tn7 site, the resulting gap can use information in the lac – heteroallele to convert the gapped region of the Tn7donor site to lac +.
Cut-and-paste transposition can be biologically replicative. The large square is a bacterial cell with two copies of the bacterial chromosome, each containing a transposable element. Transposition of the transposon from one DNA into the other results in one gapped chromosome and one chromosome containing two copies of the element, one at the donor site and one at the new site of insertion. Thus, when cells containing the bacterial chromosome with the two copies of the element are examined, transposition looks replicative, although the element actually moved by cut-and-paste transposition.
Tn7transposition can switch from a cut-and-paste mechanism to a replicative mechanism. The cut-and-paste pathway in which Tn7excises from the donor site is shown on the left. On the right in the replicative pathway, nicks occur at the 3′ ends of the transposon. These 3′ ends then attack the target DNA to form the fusion product in which the transposon links the donor and target DNAs. Repair and replication of this fusion product yield the cointegrate that contains two copies of the mobile element.
Products of intramolecular Tn7transposition. The upper line shows a linearized Tn7-containing plasmid DNA before recombination. Recombination initiates by a double-strand break at either the left end (DSB.L) or right end (DSB.R) of Tn7.The 3′-OH ends exposed by these double-strand breaks then attack the 5′ strands at the other ends of the transposon, generating species called ;“single end joins” (SEJ) that contain circularized versions of Tn7.
Tn7“;figures of eight.” The upper line shows a linearized Tn7-containing plasmid DNA before recombination. When recombination occurs with TnsAmutTnsBwt, where TnsAmut is blocked for cleavage at the 5′ ends of Tn7,only single-strand cleavage occurs at either the left or right 3′ end of Tn7.These exposed 3′ ends then execute an intramolecular attack at the 5′ strand at the other end of the transposon, generating figures of eight.
TnsC bound to the target DNA provides a platform for the transposase. The TnsAB transposase bound to the ends of Tn7is positioned on the target DNA through the interaction with TnsC. The interaction of TnsC with the transposase activates the breakage and joining events that underlie transposition. The boundaries of TnsC interaction with the target DNA are those observed in TnsCTnsD-attTn7.The transposase attacks the target site at 5′-staggered positions on the target DNA.
Modulation of TnsC activity. TnsC stimulates the transposase to execute breakage and joining when bound to ATP and the target DNA. The presence of particular targeting proteins and target DNAs determines the ATP and DNA state of TnsC.
. Positions of gain-of-function mutations in TnsC that allow recombination with TnsA +TnsB+TnsCmut. The TnsC polypeptide is indicated by the open bar; the positions of the Walker A and B motifs for purine nucleotide utilization are indicated by hatched bars. Class I gain-of-function mutations remain sensitive to target DNA signals; they are sensitive to transposition immunity and are directed to appropriate target sites by TnsD and TnsE. Class II mutants are insensitive to target signals; they promote transposition into immune targets and do not respond to TnsD and TnsE. Reprinted from reference 107 with permission.
Comparison of patterns of insertions promoted by TnsABCA225V into duplex- and triplex-containing DNA targets. (A) Transposition was performed in vitro using a duplex DNA. The transposition products were recovered by transformation into E. coliand the sequences of 100 products determined. The filled circles represent insertions in one orientation with respect to the target backbone and the open circles in the other. The hatched region is the origin-of-replication region. (B) Transposition was performed in vitro into a triplex DNA target formed by annealing a triplex-forming oligonucleotide to the plasmid and psoralen cross-linking. Insertions were recovered and mapped as described above.
Tn7insertion into E. coli attTn7.Tn7inserts about 25 bp to the 5′ side of the TnsD binding site. The TnsD binding site lies in the C-terminal region of the glmSgene and the actual point of Tn7insertion lies in the glmStranscription terminator. The middle of the 5 bp duplicated upon Tn7insertion is designated 0; sequences to the right are indicated by plus signs, and sequences to the left are indicated by a minus sign.
Positions of Tns protein binding on attTn7.The positions of TnsD and TnsC-TnsD binding on attTn7as evaluated by various footprinting methods are shown. The position of a TnsD-induced distortion in DNA at +27 is also indicated. Deletion of this region of distortion decreases TnsD and TnsCD binding to attTn7and attTn7target activity.
Target DNA structure plays a key role in Tn7insertion. Similar features of the target DNA and Tn7insertion during insertion into attTn7and adjacent to triplex DNA emphasize the critical role of target DNA structure. Target DNA distortions indicated by cross-hatching in attTn7and at triplex DNA have been detected by DNA footprinting. Reprinted from reference 62 with permission.
A model for Tn7insertion at attTn7.The binding of TnsD to attTn7results in the formation of a TnsC-TnsD-attTn7complex. The binding of TnsD results in a DNA distortion (shaded bar at +27) which attracts TnsC. TnsC that is bound in the minor groove provides a platform for the interaction and activation of the TnsAB transposase that is bound on the transposon ends. The transposase and transposon ends must attack the target DNA across the major groove.
Orientation of TnsABC+E insertions in the E. colichromosome. The position of the origin of replication (oriC) and the tersites (triangles) are marked as the two chromosomal replicores (dashed lines). The positions of 50 TnsABC+E insertions (arrows) promoted by TnsE mutants that are more active than wild-type TnsE are also marked; those outside the E. colichromosomal circle lie in one orientation with respect to the chromosome, whereas those inside are inserted in the opposite orientation. Reprinted from reference 83 with permission.
Positions of TnsABC+E insertions in the E. colichromosome. (A) A physical map of the E. colichromosome based on digestion with certain rare cutting restriction enzymes is shown. The positions of the origin of DNA replication (oriC) and several of the possible termination sites (terAto terD) are shown. (B) The distribution of 35 TnsABC+E insertions per 100 kb within the chromosome. The error bar represents the change in insertion frequency with one more or one less insertion in that region. (C) The distribution of 34 TnsABCA225V-mediated insertions that occur in the absence of TnsE.
The distribution of Tn7TnsABC+E insertions in a chromosome containing a double-strand break. A double-strand break was introduced into the bacterial chromosome by Tn10excision and the positions of Tn7insertions determined. Reprinted from reference 84 with permission.
Transposition immunity in the E. colichromosome. The mean fraction of Tn7insertion from F::Tn7 into attTn7in chromosomes containing immobile mini-Tn7ends at different distances from attTn7is expressed relative to Tn7insertion into chromosomes without a mini-Tn7element (100% = 2.3% Tn7occupancy) as assayed by Southern blotting. The contribution of background hybridization to the Tn7insertion signal in cells without a mini-Tn7element was determined from assays of cells lacking Tn7and is displayed as a dashed line. Reprinted from reference 30 with permission.
A model for Tn7transposition immunity. Tn7insertion into a target molecule results from the interaction of the TnsAB transposase bound to the Tn7element with TnsC on an appropriate target DNA (or TnsCD-attTn7or TnsCE-target DNA). A target DNA containing a Tn7end is immune to Tn7insertion because TnsB interacts with that end and with TnsC to result in the removal of TnsC from the target DNA. Reprinted from reference 106 with permission.
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