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Journal of Bacteriology, November 2003, p. 6741-6745, Vol. 185, No. 22
0021-9193/03/$08.00+0 DOI: 10.1128/JB.185.22.6741-6745.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Department of Environmental Simulation, Institute for Environmental Sciences, Rokkasho, Aomori 039-3212,1 Department of Environmental Life Sciences, Graduate School of Life Sciences, Tohoku University, Sendai 980-8577,2 Department of Applied Biochemistry, Graduate School of Agriculture and Biological Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan3
Received 6 June 2003/ Accepted 12 August 2003
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(= Tn1000), have been reported to be very crucial in the cointegration step (1, 7).
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FIG. 1. Structure of pUO1 and comparison with other IncP-1ß plasmids. The black arrowhead represents the 38-bp IR highly homologous to that of Tn21, and the white arrowhead indicates the 25-bp IR of the Tn402/Tn5053-related transposons. (A) Structure of pUO1. The white and black circles show the res sites in TnHad2 and Tn4671, respectively. The arrow indicates the transcriptional direction of the tnpA gene in IS1071. The oriV (origin of vegetative replication) and oriT (origin of plasmid transfer) sequences are shown by black boxes. The pentagon indicates the size and orientation of the gene. Black pentagons are the genes for the maintenance (replication, regulation, partition, stable inheritance) of plasmid, and shaded pentagons are those for the conjugal transfer. See references 6 and 12 for the detailed functions of the proteins encoded in these backbone regions. mer, genes for resistance to mercury; tni, genes for transposition of Tn4671. Abbreviations 1 to 10 represent the ORFs orf1 to orf10, whose functions are unclear. (B) Comparison of the four IncP-1ß plasmids. The shaded bar indicates the backbone of the IncP-1ß plasmids. The horizontal arrow shows the transcriptional direction of coding regions (A). The DNA inserts situated at the similar positions are boxed. Additional DNA insertions upstream of oriV and downstream of traM of pB4 are depicted below the backbone. Tn5720 covers Tn5719, and Tn(pB4) is a putative class II transposon that was not described in the original report (11). The 52-kb fragment on pADP-1 is flanked by two truncated versions of IS1071 (see text for details). trfA, gene for plasmid replication and regulation; atzABCDEF, genes for atrazine degradation; blaNPS-1, ß-lactamase NPS-1 gene; and mexCD and oprJ, genes for multidrug efflux system. Sequence data for R751, pADP-1, and pB4 are compiled from DDBJ/EMBL/GenBank databases: R751, U67194; pADP-1, U66917; and pB4, AJ431260. (C) Comparison of the insertion sites of transposons and other phenotypic genes. The figures represent the oriV-trfA and trb-tra regions of pUO1, and the insertion sites of transposon orvarious phenotypic genes are drawn so as to locate on the pUO1 backbone. Vertical arrowhead indicates the insertion site of the foreign DNA, and the box represents the conserved 20-bp IR sequence (see text for details) on pUO1. This 20-bp sequence has been reported to be present in the corresponding regions on the three other plasmids (the oriV-trfA region: five copies on R751 and four copies on pADP-1 and pB4; and the trb-tra region: four and five copies on R751 and pB4, respectively) (6, 11, 12).
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FIG. 2. Schematic structures of TnHad2 and a mini-TnHad2 derivative. (A) Structure of TnHad2. The sequences of the three IRs are shown below the figure, and the dot indicates the nucleotide identical to that of IR1. The 38-bp IR2 sequence redefined in this study is depicted in the figure, and the IR2 sequence in our previous report (10) had been defined to the 37-bp sequence that corresponds to base positions from 2 to 38 in this figure. This revision is based on our observation in this study that the first C residue of IR2 was incorporated into the fragment that transposed onto a target replicon. In this study, the rightmost 818-bp fragment of TnHad2 containing IR1, IR2, and the res site is designated mini-TnHad2. (B) Structure of the mini-TnHad2 derivative on pMS015. The details to construct this plasmid have been described previously (10). Note that the IR2 derivative on this plasmid has the T, but not C, residue at the first position.
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, whose transposases are not able to catalyze the transposition of the Tn21-related transposons (3, 9).
The Escherichia coli strains DH5
and HB101 (2) used in this study were cultivated at 37°C in Luria broth (LB) and LB agar. The following agents were added to the media: chloramphenicol, 50 µg/ml; kanamycin, 50 µg/ml; streptomycin, 50 µg/ml; and sulfathiazole, 350 µg/ml. Established protocols were employed for the preparation of plasmid DNA, DNA digestion with restriction endonucleases, ligation, gel electrophoresis, transformation of E. coli cells, DNA sequencing, and PCR (2).
Genetic organization of pUO1. To understand the basic properties of pUO1, its complete sequence with a size of 67,066 bp (accession no. AB063332 in DDBJ/EMBL/GenBank databases) was determined by standard methods (2). Computer analysis with the software programs GENETYX 10 (SDC, Inc., Tokyo, Japan) and BLAST 2 (National Institute of Genetics, Mishima, Japan) led to identification of 70 putative open reading frames (ORFs) on pUO1 (Fig. 1A). Homology searches clarified that pUO1 was a member of the IncP-1ß plasmids, since the backbone regions of pUO1 encoding its replication, maintenance, and transfer functions and their regulation shared more than 99% nucleotide identity with the corresponding segments of two well-characterized broad-host-range IncP-1ß plasmids, a trimethoprim resistance plasmid, R751 (12), and an atrazine-catabolic plasmid, pADP-1 (6). The mer genes on pUO1 were located within two Tn402/Tn5053-related transposons designated Tn4671 (8,448 bp) and Tn4672 (11,914 bp), and both elements shared one end (Fig. 1A). The Tn4671 sequence was quite similar (92%) to that of an 8,447-bp mercury resistance transposon, Tn5053, on the chromosome of Xanthomonas sp. strain W17 (4). We were able to detect the intramolecular transposition of Tn4671 in pUO1 (data not shown). The Tn4672 region outside of Tn4671 carried six putative ORFs (orf6 to orf10 and invA) (Fig. 1A). While the predicted invA product with a size of 185 amino acid residues showed >91% identity to a hypothetical DNA invertase from Rhodospirillum rubrum S-1 (accession no. D17434) and two putative invertases from a drug resistance IncP-1ß plasmid, pB4 (accession no. AJ431260) (11), no functions of the orf6 to orf10 products were defined by the database searches. The Tn402/Tn5053-related transposons have been reported to generate a 5-bp duplication of the target sequence upon transposition (4). Neither Tn4671 nor Tn4672 had such duplications at their external junctions, indicating that the putative duplication might have been lost by unknown DNA rearrangements.
Comparison of pUO1 with three other completely sequenced IncP-1ß plasmids, R751 (12), pADP-1 (6), and pB4 (11), revealed that all four of the plasmids had
foreign
DNA fragments (i.e., transposons and various phenotypic genes) at very close positions between oriV and trfA and between the trb and tra clusters on the IncP-1ß backbone (Fig. 1B and C). The DNA inserts in the oriV-trfA regions on these plasmids had the Tn21-like 38-bp IR sequences at one or both ends (Fig. 1B). It is therefore most likely that the insertion of the Tn21-related transposons or their ancestors into the oriV-trfA region of a common plasmid must have led to physical separation of the IncP-1ß backbone. The DNA inserts in the trb-tra region on the four plasmids had the Tn402/Tn5053-related transposons or their remnants (Fig. 1B). It is noteworthy that these inserts on pUO1 and pB4 were situated at an identical position on the backbone and that this position was only 3 bp apart from the insertion site of Tn402 on R751 (data not shown) (Fig. 1C). We noticed that the right and left ends of the 52-kb insert on pADP-1 carried the terminal 68- and 476-bp sequences, respectively, of IS1071, indicating that the 52-kb insert might have been present as a huge IS1071-composite transposon on the ancestor of this plasmid. Thorsted et al. (12) and two other groups (6, 11) have proposed that several copies of a conserved 20-bp inverted repeat sequence (consensus = CATCGCCANNTCYGRCGATG) residing in the oriV-trfA and trb-tra regions on R751, pB4, and pADP-1 might have been associated with the insertion of the foreign DNA fragments. pUO1 also had four and five copies of this conserved sequence in the former and latter regions, respectively (Fig. 1C), supporting the proposal by Thorsted et al. and others and providing a better understanding for the evolution and establishment of various IncP-1ß plasmids.
Transposition of the mini-TnHad2 derivatives.
As described above, the four outermost G residues in the IRs are conserved in the class II transposons (3). However, the first 5' position of IR2 of mini-TnHad2 was occupied by the C residue, generating the 5'-CGGG end (Fig. 2A) (10). We have previously replaced this C residue with the T residue, and this mutant version of mini-TnHad2 with an additional insert of a kanamycin resistance (Kmr) gene was cloned into a p15A-based Cmr vector, pSTV29 (TAKARA BIO), to obtain pMS015 (Fig. 2B) (10). The mini-TnHad2 derivative on pMS015 has been found to still transpose efficiently in the presence of the Tn21 transposase, generating the transposed fragment having IR1 (5'-GGGG end) and the mutant IR2 (5'-TGGG end) at the extremities (10). This observation has provided us a possibility that the Tn21 transposase might be able to catalyze the transposition of the mini-TnHad2 derivatives that have various mutations in the four outermost positions of the IRs. To investigate this possibility in this study, additional mini-TnHad2 derivatives with various mutations in the four outermost residues of IR1 and IR2 were constructed by PCR using pMS015 as the template and various IR1- and IR2-based sequences as the primers. The PCR products were cloned in pSTV29 or its related vector, pSTV28, to obtain the plasmids listed in Table 1; the details to construct these plasmids will be provided upon request. Each of these plasmids was introduced into a DH5
derivative harboring R388 (16), a conjugal and Sur plasmid free of transposons, and pMT1252 (10), a pBR322 derivative carrying the transposition genes of Tn21. The DH5
derivative carrying the three plasmids was used as a donor to mate with an Smr strain, HB101, on a membrane filter, and the transposition of the mini-TnHad2 derivatives onto R388 was, as described previously (10), examined by selecting and analyzing the Kmr Smr transconjugants.
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TABLE 1. Transposition of the mini-TnHad2 derivatives with the mutant end(s)a
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led to the complete abolishment of transposition (7). To know whether the critical roles of the second and third G residues of the 
IRs in transposition were the cases with the IRs of mini-TnHad2, we first introduced various mutations into the second position of both IRs of mini-TnHad2 to obtain the three derivative transposons that carried the two 5'-GAGG, 5'-GTGG, and 5'-GCGG ends on pMS0196, pMS0197, and pMS0192, respectively (Table 1, experiments 6 to 8). The mini-TnHad2 derivative on pMS0196 (5'-GAGG) transposed at the frequency 800-fold lower than that on pMS015G, and the two transposons on pMS0197 (5'-GTGG) and pMS0192 (5'-GCGG) did not transpose at all. Replacement of the second C residue of the pMS0192-specified IR1 mutant with the G residue gave rise to pMS0195 (Table 1, experiment 9), and this mini-TnHad2 derivative with the 5-GGGG and 5'-GCGG ends exhibited the transposability at the frequency only 50-fold lower than that on pMS015G. These results indicated that (i) the presence of the A, but not T or C, residue at the second position in both IRs still permitted the transposition albeit at the much lower frequency, and (ii) the presence of the G residue at the second position at least in one IR allowed the efficient transposition. To examine the role of the third G residue in the IR, we next constructed pMS0193 in which both IRs of mini-TnHad2 were changed to the 5'-GGCG ends (Table 1, experiment 10). The resulting mini-TnHad2 derivative transposed at the frequency 500-fold lower than that on pMS015G. This was a unique property associated with mini-TnHad2, because a mutant of 
having the two 5'-GGTG ends has been reported not to be transposable at all (7). Taking into consideration that the fourth G residue is conserved in the IRs of the class II transposons (3, 9), we also investigated the importance of the fourth G residue in the mini-TnHad2 IRs. The mini-TnHad2 derivative having the two 5'-GGGC ends on pMS0194 transposed at the frequency 150-fold lower than that on pMS015G (Table 1, experiment 11), demonstrating that the transposition of mini-TnHad2 did not absolutely require the presence of the G residue at the fourth position in both IRs. On the basis of (i) the efficient transposition of the mini-TnHad2 derivative with the two 5'-CGGG ends, (ii) the inefficient but clearly detectable transposition of the derivative with the two 5'-GGCG or 5'-GGGC ends, and (iii) no transposition of the derivative with the two 5'-GCGG ends, we constructed pMS0198 in which the mini-TnHad2 derivative was flanked by the two 5'-CGCC ends (Table 1, experiment 12). The resulting transposon having these ends still moved at the frequency 2,800-fold lower than that on pMS015G, and this result reinforced the unique characteristics of mini-TnHad2 in that the presence of the G residue at the second positions was critical for transposition. It has been reported that the Tn21 transposase was able to catalyze the cointegration reaction even when its inner 20-bp sequence of the IR was replaced by that of Tn3 (5). This nonstrict property of the Tn21 transposase might also be associated with its recognition of the four outermost residues of mini-TnHad2, hence leading to our successful detection of the transposition of the mini-TnHad2 derivative having the 5'-CGCC end in both IRs. The drastic decrease in the transposition frequencies of several mini-TnHad2 mutants might be ascribed to either or both (i) inefficient binding of the Tn21 transposase to the mutant IRs and (ii) inefficient but still successful single-stranded break of the 3' ends of the mini-TnHad2 mutants by the Tn21 transposase in the initial step of the cointegration reaction (3, 9). Biochemical purification of the Tn21 transposase and subsequent in vitro assay of its binding to various mutant IRs of mini-TnHad2 will elucidate more detailed interaction of the transposase with the IR.
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transposable element. J. Mol. Biol. 247:578-587.[CrossRef][Medline]
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