We report the complete genome sequence of Acidovorax sp. strain KKS102, a polychlorinated-biphenyl-degrading strain isolated from a soil sample in Tokyo. The genome contains a single circular 5,196,935-bp chromosome and no plasmids.
A polychlorinated biphenyl (PCB)/biphenyl degradation gene cluster in Acidovorax sp. strain KKS102, which is very similar to that in Tn4371 from Cupriavidus oxalaticus A5, was transferred to several proteobacterial strains by conjugation. The mobilized DNA fragment consisted of 61,807 bp and carried genes for mating-pair formation (mpf), DNA transfer (dtr), integrase (int), and replication-partition proteins (rep-parAB). In the transconjugants, transferred DNA was integrated at ATTGCATCAG or similar sequences. The circular-form integrative and conjugative element (ICE) was detected by PCR, and quantitative PCR analyses revealed that, in KKS102 cells, the ratio of the circular form to the integrated form was very low (approximately 10−5). The circular form was not detected in a mutant of the int gene, which was located at the extreme left and transcribed in the inward direction, and the level of int transcriptional activity was much higher in the circular form than in the integrated form. These findings clearly demonstrated that the genes for PCB/biphenyl degradation in KKS102 cells are located on an ICE, which was named ICEKKS1024677. Comparisons of similar ICE-like elements collected from the public database suggested that those of beta- and gammaproteobacteria were distinguishable from other ICE-like elements, including those in alphaproteobacteria, with respect to the gene composition and gene organization.
Sphingobium japonicum strain UT26 utilizes γ-hexachlorocyclohexane (γ-HCH), a man-made chlorinated pesticide that causes serious environmental problems due to its toxicity and long persistence, as a sole source of carbon and energy. Here, we report the complete genome sequence of UT26, which consists of two chromosomes and three plasmids. The 15 lin genes involved in γ-HCH degradation are dispersed on the two chromosomes and one of the three plasmids.
To understand the mechanisms for structural diversification of Pseudomonas-derived toluene-catabolic (TOL) plasmids, the complete sequence of a self-transmissible plasmid pDK1 with a size of 128,921 bp from Pseudomonas putida HS1 was determined. Comparative analysis revealed that (i) pDK1 consisted of a 75.6-kb IncP-7 plasmid backbone and 53.2-kb accessory gene segments that were bounded by transposon-associated regions, (ii) the genes for conjugative transfer of pDK1 were highly similar to those of MOBH group of mobilizable plasmids, and (iii) the toluene-catabolic (xyl) gene clusters of pDK1 were derived through homologous recombination, transposition, and site-specific recombination from the xyl gene clusters homologous to another TOL plasmid, pWW53. The minireplicons of pDK1 and its related IncP-7 plasmids, pWW53 and pCAR1, that contain replication and partition genes were maintained in all of six Pseudomonas strains tested, but not in alpha- or betaproteobacterial strains. The recipient host range of conjugative transfer of pDK1 was, however, limited to two Pseudomonas strains. These results indicate that IncP-7 plasmids are essentially narrow-host-range and self-transmissible plasmids that encode MOBH group-related transfer functions and that the host range of IncP-7-specified conjugative transfer was, unlike the situation in other well-known plasmids, narrower than that of its replication.
A haloalkane dehalogenase, DbjA, was crystallized by the hanging-drop vapour-diffusion method using PEG 4000 as a precipitant. The crystal belongs to the orthorhombic system, space group P21212 and diffracts to 1.75 Å resolution.
Haloalkane dehalogenases are key enzymes for the degradation of halogenated aliphatic pollutants. The haloalkane dehalogenase DbjA constitutes a novel substrate-specificity class with high catalytic activity for β-methylated haloalkanes. In order to reveal the mechanism of its substrate specificity, DbjA has been crystallized using the hanging-drop vapour-diffusion method. The best crystals were obtained using the microseeding technique with a reservoir solution consisting of 17–19.5%(w/v) PEG 4000, 0.2 M calcium acetate and 0.1 M Tris–HCl pH 7.7–8.0. The space group of the DbjA crystal is P21212, with unit-cell parameters a = 212.9, b = 117.8, c = 55.8 Å. The crystal diffracts to 1.75 Å resolution.
haloalkane dehalogenases; biodegradation; α/β hydrolases; rhizobia
Pseudomonas putida G7 carries a naphthalene-catabolic and self-transmissible plasmid, NAH7, which belongs to the IncP-9 incompatibility group. Adjacent to the putative origin of conjugative transfer (oriT) of NAH7 are three genes, traD, traE, and traF, whose functions and roles in conjugation were previously unclear. These three genes were transcribed monocistronically and thus were designated the traD operon. Mutation of the three genes in the traD operon resulted in 10- to 105-fold decreases in the transfer frequencies of the plasmids from Pseudomonas to Pseudomonas and Escherichia coli and from E. coli to E. coli. On the other hand, the traD operon was essential for the transfer of NAH7 from E. coli to Pseudomonas strains. These results indicated that the traD operon is a host-range modifier in the conjugative transfer of NAH7. The TraD, TraE, and TraF proteins were localized in the cytoplasm, periplasm, and membrane, respectively, in strain G7 cells. Our use of a bacterial two-hybrid assay system showed that TraE interacted in vivo with other essential components for conjugative transfer, including TraB (coupling protein), TraC (relaxase), and MpfH (a channel subunit in the mating pair formation system).
Rhizobia are nitrogen-fixing soil bacteria that establish endosymbiosis with some leguminous plants. The completion of several rhizobial genome sequences provides opportunities for genome-wide functional studies of the physiological roles of many rhizobial genes. In order to carry out genome-wide phenotypic screenings, we have constructed a large mutant library of the nitrogen-fixing symbiotic bacterium, Mesorhizobium loti, by transposon mutagenesis. Transposon insertion mutants were generated using the signature-tagged mutagenesis (STM) technique and a total of 29 330 independent mutants were obtained. Along with the collection of transposon mutants, we have determined the transposon insertion sites for 7892 clones, and confirmed insertions in 3680 non-redundant M. loti genes (50.5% of the total number of M. loti genes). Transposon insertions were randomly distributed throughout the M. loti genome without any bias toward G+C contents of insertion target sites and transposon plasmids used for the mutagenesis. We also show the utility of STM mutants by examining the specificity of signature tags and test screenings for growth- and nodulation-deficient mutants. This defined mutant library allows for genome-wide forward- and reverse-genetic functional studies of M. loti and will serve as an invaluable resource for researchers to further our understanding of rhizobial biology.
Mesorhizobium loti; signature-tagged mutagenesis; mutant library; reverse genetics
The number of available genome sequences is increasing, and easy-to-use software that enables efficient comparative analysis is needed.
We developed GenomeMatcher, a stand-alone software package for Mac OS X. GenomeMatcher executes BLAST and MUMmer, and the detected similarities are displayed in two-dimensional and parallel views with similarity values indicated by color. Selection and re-computation of any subregions is easily performed and allows flexible and in-depth analysis. Furthermore, symbols for annotation data are displayed along the views, and the user can relate the genomic differences with annotation data. While bl2seq allows sub-Giga base comparison, three alignment programs, bl2seq, MAFFT and ClustalW, together with a dotmatch program allow comparative analysis of single-nucleotide level resolution. GenomeMatcher images can be saved as PDF and TIFF files for presentation. As examples of graphical ability of GenomeMatcher to show similarity in colors, we show two cases in Burkholderia and Vivrio strains that the nucleotide sequence of the second largest chromosome changes more rapidly than the largest chromosome.
GenomeMatcher is efficient and easy-to-use stand-alone software for in-depth comparative analysis of two sequences. GenomeMatcher is useful for detecting similarities in DNA sequences ranging in size from a few to sub-Giga bases.
Sphingobium japonicum UT26 utilizes γ-hexachlorocyclohexane (γ-HCH) as its sole source of carbon and energy. In our previous studies, we cloned and characterized genes encoding enzymes for the conversion of γ-HCH to β-ketoadipate in UT26. In this study, we analyzed a mutant obtained by transposon mutagenesis and identified and characterized new genes encoding a putative ABC-type transporter essential for the utilization of γ-HCH in strain UT26. This putative ABC transporter consists of four components, permease, ATPase, periplasmic protein, and lipoprotein, encoded by linK, linL, linM, and linN, respectively. Mutation and complementation analyses indicated that all the linKLMN genes are required, probably as a set, for γ-HCH utilization in UT26. Furthermore, the mutant cells deficient in this putative ABC transporter showed (i) higher γ-HCH degradation activity and greater accumulation of the toxic dead-end product 2,5-dichlorophenol (2,5-DCP), (ii) higher sensitivity to 2,5-DCP itself, and (iii) higher permeability of hydrophobic compounds than the wild-type cells. These results strongly suggested that LinKLMN are involved in γ-HCH utilization by controlling membrane hydrophobicity. This study clearly demonstrated that a cellular factor besides catabolic enzymes and transcriptional regulators is essential for utilization of xenobiotic compounds in bacterial cells.
The overall architecture of IncP-1 plasmids is very conserved in that the accessory genes are typically located in one or two specific regions: between oriV and trfA and between the tra and trb operons. Various hypotheses have been formulated to explain this, but none have been tested experimentally. We investigated whether this structural similarity is due to region-specific transposition alone or also is reliant on selection for plasmids with insertions limited to these two regions. We first examined the transposition of Tn21Km into IncP-1β plasmid pBP136 and found that most Tn21Km insertions (67%) were located around oriV. A similar experiment using the oriV region of IncP-1β plasmid pUO1 confirmed these results. We then tested the transferability, stability, and fitness cost of different pBP136 derivatives to determine if impairment of these key plasmid characters explained the conserved plasmid architecture. Most of the pBP136 derivatives with insertions in transfer genes were no longer transferable. The plasmids with insertions in the oriV-trfA and tra-trb regions were more stable than other plasmid variants, and one of these also showed a significantly lower fitness cost. In addition, our detailed sequence analysis of IncP-1 plasmids showed that Tn402/5053-like transposons are situated predominantly between the tra and trb operons and close to the putative resolution site for the ParA resolvase, a potential hot spot for those transposons. Our study presents the first empirical evidence that region-specific insertion of transposons in combination with selection for transferable and stable plasmids explains the structural similarity of IncP-1 plasmids.
The α-proteobacterial strain Sphingobium japonicum UT26 utilizes a highly chlorinated pesticide, γ-hexachlorocyclohexane (γ-HCH), as a sole source of carbon and energy, and haloalkane dehalogenase LinB catalyzes the second step of γ-HCH degradation in UT26. Functional complementation of a linB mutant of UT26, UT26DB, was performed by the exogenous plasmid isolation technique using HCH-contaminated soil, leading to our successful identification of a plasmid, pLB1, carrying the linB gene. Complete sequencing analysis of pLB1, with a size of 65,998 bp, revealed that it carries (i) 50 totally annotated coding sequences, (ii) an IS6100 composite transposon containing two copies of linB, and (iii) potential genes for replication, maintenance, and conjugative transfer with low levels of similarity to other homologues. A minireplicon assay demonstrated that a 2-kb region containing the predicted repA gene and its upstream region of pLB1 functions as an autonomously replicating unit in UT26. Furthermore, pLB1 was conjugally transferred from UT26DB to other α-proteobacterial strains but not to any of the β- or γ-proteobacterial strains examined to date. These results suggest that this exogenously isolated novel plasmid contributes to the dissemination of at least some genes for γ-HCH degradation in the natural environment. To the best of our knowledge, this is the first detailed report of a plasmid involved in γ-HCH degradation.
The naphthalene-catabolic (nah) genes on the incompatibility group P-9 (IncP-9) self-transmissible plasmid NAH7 from Pseudomonas putida G7 are some of the most extensively characterized genetic determinants for bacterial aerobic catabolism of aromatic hydrocarbons. In contrast to the detailed studies of its catabolic cascade and enzymatic functions, the biological characteristics of plasmid NAH7 have remained unclear. Our sequence determination in this study together with the previously deposited sequences revealed the entire structure of NAH7 (82,232 bp). Comparison of NAH7 with two other completely sequenced IncP-9 catabolic plasmids, pDTG1 and pWW0, revealed that the three plasmids share very high nucleotide similarities in a 39-kb region encoding the basic plasmid functions (the IncP-9 backbone). The backbone of NAH7 is phylogenetically more related to that of pDTG1 than that of pWW0. These three plasmids carry their catabolic gene clusters at different positions on the IncP-9 backbone. All of the NAH7-specified nah genes are located on a class II transposon, Tn4655. Our analysis of the Tn4655-encoded site-specific recombination system revealed that (i) a novel tyrosine recombinase, TnpI, catalyzed both the intra- and intermolecular recombination between two copies of the attI site, (ii) the functional attI site was located within a 119-bp segment, and (iii) the site-specific strand exchange occurred within a 30-bp segment in the 41-bp CORE site. Our results and the sequence data of other naphthalene-catabolic plasmids, pDTG1 and pND6-1, suggest a potential role of the TnpI-attI recombination system in the establishment of these catabolic plasmids.
Isolated from Pseudomonas resinovorans CA10, pCAR1 is a 199-kb plasmid that carries genes involved in the degradation of carbazole and dioxin. The nucleotide sequence of pCAR1 has been determined previously. In this study, we characterized pCAR1 in terms of its replication, maintenance, and conjugation. By constructing miniplasmids of pCAR1 and testing their establishment in Pseudomonas putida DS1, we show that pCAR1 replication is due to the repA gene and its upstream DNA region. The repA gene and putative oriV region could be separated in P. putida DS1, and the oriV region was determined to be located within the 345-bp region between the repA and parW genes. Incompatibility testing using the minireplicon of pCAR1 and IncP plasmids indicated that pCAR1 belongs to the IncP-7 group. Monitoring of the maintenance properties of serial miniplasmids in nonselective medium, and mutation and complementation analyses of the parWABC genes, showed that the stability of pCAR1 is attributable to the products of the parWAB genes. In mating assays, the transfer of pCAR1 from CA10 was detected in a CA10 derivative that was cured of pCAR1 (CA10dm4) and in P. putida KT2440 at frequencies of 3 × 10−1 and 3 × 10−3 per donor strain, respectively. This is the first report of the characterization of this completely sequenced IncP-7 plasmid.
Various xenobiotic-degrading genes on many catabolic plasmids are often flanked by two copies of an insertion sequence, IS1071. This 3.2-kb IS element has long (110-bp) terminal inverted repeats (IRs) and a transposase gene that are phylogenetically related to those of the class II transposons. However, the transposition mechanism of IS1071 has remained unclear. Our study revealed that IS1071 was only able to transpose at high frequencies in two environmental β-proteobacterial strains, Comamonas testosteroni and Delftia acidovorans, and not in any of the bacteria examined which belong to the α- and γ-proteobacteria. IS1071 was found to have the functional features of the class II transposons in that (i) the final product of the IS1071 transposition was a cointegrate of its donor and target DNA molecules connected by two directly repeated copies of IS1071, one at each junction; (ii) a 5-bp duplication of the target sequence was observed at the insertion site; and (iii) a tnpA mutation of IS1071 was efficiently complemented by supplying the wild-type tnpA gene in trans. Deletion analysis of the IS1071 IR sequences indicated that nearly the entire region of the IRs was required for its transposition, suggesting that the interaction between the transposase and IRs of IS1071 might be different from that of the other well-characterized class II transposons.
Haloalkane dehalogenases are key enzymes for the degradation of halogenated aliphatic pollutants. Two rhizobial strains, Mesorhizobium loti MAFF303099 and Bradyrhizobium japonicum USDA110, have open reading frames (ORFs), mlr5434 and blr1087, respectively, that encode putative haloalkane dehalogenase homologues. The crude extracts of Escherichia coli strains expressing mlr5434 and blr1087 showed the ability to dehalogenate 18 halogenated compounds, indicating that these ORFs indeed encode haloalkane dehalogenases. Therefore, these ORFs were referred to as dmlA (dehalogenase from Mesorhizobium loti) and dbjA (dehalogenase from Bradyrhizobium japonicum), respectively. The principal component analysis of the substrate specificities of various haloalkane dehalogenases clearly showed that DbjA and DmlA constitute a novel substrate specificity class with extraordinarily high activity towards β-methylated compounds. Comparison of the circular dichroism spectra of DbjA and other dehalogenases strongly suggested that DbjA contains more α-helices than the other dehalogenases. The dehalogenase activity of resting cells and Northern blot analyses both revealed that the dmlA and dbjA genes were expressed under normal culture conditions in MAFF303099 and USDA110 strain cells, respectively.
An efficient and quantitative method to analyze the transposition of various insertion sequence (IS) elements in Burkholderia multivorans ATCC 17616 was devised. pGEN500, a plasmid carrying a Bacillus subtilis-derived sacB gene, was introduced into ATCC 17616 cells, and 25% of their sucrose-resistant derivatives were found to carry various IS elements on pGEN500. A PCR-based experimental protocol, in which a mixture of several specific primer pairs was used, revealed that pGEN500 captured, in addition to five previously reported IS elements (IS401, IS402, IS406, IS407, and IS408), three novel IS elements, ISBmu1, ISBmu2, and ISBmu3. The global transposition frequency of these IS elements was enhanced more than sevenfold under a high-temperature condition (42°C) but not under oxidative stress or starvation conditions. To our knowledge, this is the first report demonstrating the elevated transposition activities of several IS elements at a high temperature. The efficient experimental protocol developed in this study will be useful in quantitatively and simultaneously investigating various IS elements, as well as in capturing novel functional mobile elements from a wide variety of bacteria.
β-Hexachlorocyclohexane (β-HCH) is the most recalcitrant among the α-, β-, γ-, and δ-isomers of HCH and causes serious environmental pollution problems. We demonstrate here that the haloalkane dehalogenase LinB, reported earlier to mediate the second step in the degradation of γ-HCH in Sphingomonas paucimobilis UT26, metabolizes β-HCH to produce 2,3,4,5,6-pentachlorocyclohexanol.
Sphingomonas paucimobilis UT26 utilizes γ-hexachlorocyclohexane (γ-HCH) as a sole source of carbon and energy. In our previous study, we cloned and characterized genes that are involved in the conversion of γ-HCH to maleylacetate (MA) via chlorohydroquinone (CHQ) in UT26. In this study, we identified and characterized an MA reductase gene, designated linF, that is essential for the utilization of γ-HCH in UT26. A gene named linEb, whose deduced product showed significant identity to LinE (53%), was located close to linF. LinE is a novel type of ring cleavage dioxygenase that catalyzes the conversion of CHQ to MA. LinEb expressed in Escherichia coli transformed CHQ and 2,6-dichlorohydroquinone to MA and 2-chloromaleylacetate, respectively. Our previous and present results indicate that UT26 (i) has two gene clusters for degradation of chlorinated aromatic compounds via hydroquinone-type intermediates and (ii) uses at least parts of both clusters for γ-HCH utilization.
The self-transmissible plasmid pUO1 from Delftia acidovorans strain B carries two haloacetate-catabolic transposons, TnHad1 and TnHad2, and the mer genes for resistance to mercury. The complete 67,066-bp sequence of pUO1 revealed that the mer genes were also carried by two Tn402/Tn5053-like transposons, Tn4671 and Tn4672, and that the pUO1 backbone regions shared 99% identity to those of the archetype IncP-1β plasmid R751. Comparison of pUO1 with three other IncP-1β plasmids illustrated the importance of transposon insertion in the diversity and evolution of this group of plasmids. Mutational analysis of the four outermost residues in the inverted repeats (IRs) of TnHad2, a Tn21-related transposon, revealed a crucial role of the second residue of its IRs in transposition.
The Burkholderia multivorans strain ATCC 17616 carries three circular chromosomes with sizes of 3.4, 2.5, and 0.9 Mb. To determine the distribution and organization of the amino acid biosynthetic genes on the genome of this β-proteobacterium, various auxotrophic mutations were isolated using a Tn5 derivative that was convenient not only for the determination of its insertion site on the genome map but also for the structural analysis of the flanking regions. Analysis by pulsed-field gel electrophoresis revealed that 20 out of 23 insertion mutations were distributed on the 3.4-Mb chromosome. More detailed analysis of the his, trp, arg, and lys mutations and their flanking regions revealed the following properties of these auxotrophic genes: (i) all nine his genes were clustered on the 3.4-Mb chromosome; (ii) seven trp genes were organized within two distinct regions, i.e., a trpEGDC cluster on the 3.4-Mb chromosome and a trpFBA cluster on the 2.5-Mb chromosome; (iii) the leu gene cluster, leuCDB, was also located close to the trpFBA cluster; and (iv) lysA and argG genes were located on the 2.5-Mb chromosome, in contrast to the argH gene, which was located on the 3.4-Mb chromosome. Southern hybridization analysis, allelic exchange mutagenesis of ATCC 17616, and complementation tests demonstrated that all of the genes examined were functional and existed as a single copy within the genome. The present findings also indicated that the 2.5-Mb chromosome carried various auxotrophic genes with no structural or functional counterparts on the remaining two chromosomes.
The homology model of protein Rv2579 from Mycobacterium tuberculosis H37Rv was compared with the crystal structure of haloalkane dehalogenase LinB from Sphingomonas paucimobilis UT26, and this analysis revealed that 6 of 19 amino acid residues which form an active site and entrance tunnel are different in LinB and Rv2579. To characterize the effect of replacement of these six amino acid residues, mutations were introduced cumulatively into the six amino acid residues of LinB. The sixfold mutant, which was supposed to have the active site of Rv2579, exhibited haloalkane dehalogenase activity with the haloalkanes tested, confirming that Rv2579 is a member of the haloalkane dehalogenase protein family.
The 56-kb class II toluene catabolic transposon Tn4651 from Pseudomonas putida plasmid pWW0 is unique in that (i) its efficient resolution requires, in addition to the 0.2-kb resolution (res) site, the two gene products TnpS and TnpT and (ii) the 2.4-kb tnpT-res-tnpS region is 48 kb apart from the tnpA gene (M. Tsuda, K.-I. Minegishi, and T. Iino, J. Bacteriol. 171:1386-1393, 1989). Detailed analysis of the 2.4-kb region revealed that the tnpS and tnpT genes encoding the putative 323- and 332-amino-acid proteins, respectively, were transcribed divergently with an overlapping 59-bp sequence in the 203-bp res site. The motifs (the R-H-R-Y tetrad in domains I and II with proper spacing) commonly conserved in the integrase family of site-specific recombinases were found in TnpS. In contrast, TnpT did not show any significant amino acid sequence homology to the other proteins that are directly or indirectly involved in recombination. Analysis of site-specific recombination under the Escherichia coli recA cells indicated that (i) the site-specific resolution between the two copies of the res site on a single molecule was catalyzed by TnpS, (ii) the functional res site was located within a 95-bp segment, and (iii) TnpT appeared to have the role of enhancing the site-specific resolution. It was also found that TnpS catalyzed the site-specific recombination between the res sites located at two different molecules to form a cointegrate molecule. Site-specific mutagenesis of the conserved tyrosine residue in TnpS led to the loss of both the resolution and the integration activities, indicating that such a residue took part in both types of recombination.
The two haloacetate dehalogenase genes, dehH1 and dehH2, on the 65-kb plasmid pUO1 from Delftia acidovorans strain B were found to be located on transposable elements. The dehH2 gene was carried on an 8.9-kb class I composite transposon (TnHad1) that was flanked by two directly repeated copies of IS1071, IS1071L and IS1071R. The dehH1 gene was also flanked by IS1071L and a truncated version of IS1071 (IS1071N). TnHad1, dehH1, and IS1071N were located on a 15.6-kb class II transposon (TnHad2) whose terminal inverted repeats and res site showed high homology with those of the Tn21-related transposons. TnHad2 was defective in transposition because of its lacking the transposase and resolvase genes. TnHad2 could transpose when the Tn21-encoded transposase and resolvase were supplied in trans. These results demonstrated that Tn Had2 is a defective Tn21-related transposon carrying another class I catabolic transposon.
It has been reported that the toluene-degrading (xyl) genes from Pseudomonas putida plasmid pWW53 are able to translocate to broad-host-range drug resistance plasmid RP4, and pWW53-4 is one of the smallest RP4 derivatives (H. Keil, S. Keil, R. W. Pickup, and P. A. Williams, J. Bacteriol. 164:887–895, 1985). Our investigation of pWW53-4 in this study demonstrated that such a translocated region that is 39 kb long is a transposon. This mobile element, Tn4656, was classified as a class II transposon since its transposition occurred by a two-step process: transposase (TnpA)-mediated formation of the cointegrate and resolvase (TnpR)-mediated site-specific resolution of the cointegrate at the two copies of the res site. The Tn4656 TnpA and TnpR functions encoded in the rightmost 4-kb region were found to be exchangeable with those specified by other Tn1721-related class II transposons, including another toluene transposon, Tn4653. Sequence analysis of the transposition-related genes and sites of Tn4656 also supported the hypothesis that this transposon is closely related to the Tn1721-related transposons. The lower transposition frequency of Tn4656 has been suggested to be due to the unique nucleotide sequence of one of the terminal 39-bp inverted repeats.