Intermolecular and intramolecular transposition and transposition immunity in Tn3 and Tn2660

Summary Intermolecular transposition of Tn2660 into pCR1 was measured at 30°C in recA ^– and recA ⁺ hosts as between 2.6 and 5.5x10^–3, a similar value to that previously found for Tn3. No cointegrate structures were found under conditions where 10⁴ transposition events occurred. Immunity to intermolecular transposition of Tn2660, similar to that found for Tn3 was demonstrated by showing that the above transposition frequency was reduced by a factor of between 10^–3 and 10^–4 when a mutant Tn2660 (resulting in the synthesis of a temperaturesensitive -lactamase) was present in the recipient plasmid. Intramolecular transposition of Tn3 was found to occur under the same conditions as previously demonstrated for Tn2660 giving rise to similar end products, in which the newly introduced Tn3 is oriented inversely to the resident Tn3 and the DNA sequence between the two transposons has been inverted. Thus, in all respects functional identity of the transposition activities of Tn3 and Tn2660 is shown, thereby identifying characteristics of intramolecular transposition that are not readily accommodated by current models of transposition.     

Does Tn10 transpose via the cointegrate molecule?

Summary It has been well established that Tn3 and its relatives transpose from one replicon to another by two successive reactions: formation of the cointegrate molecule and resolution from it. Whether or not the 9300 base pair tetracycline resistance transposon Tn10 transposes in the same manner as Tn3 was investigated by two methods.In the first method, 55, a lambda phage carrying Tn10 was lysogenized in an Escherichia coli strain carrying a Tn10 insertion; the phage has a deletion in attP, hence it was lysogenized in a Tn10 sequence in the E. coli chromosome by reciprocal recombination. The chromosomal structure in these lysogens is equivalent to the Tn10-mediated cointegrate molecule of lambda and the E. coli chromosomal DNA. The stability of the cointegrate molecule was examined by measuring the rate of excision of lambda from the host chromosome, and was found to be stable, especially in a Rec^- strain. Because of this stability, the cointegrate molecule should be accumulated if Tn10 transposes via the cointegrate molecule. Then, we examined the configuration of products made by transposition of Tn10 from 55 to the E. coli chromosome. The cointegrate molecule was found in products of Tn10 transposition in a Rec⁺ strain at a frequency of 5% per Tn10 transposition, but this molecule could not be found in a Rec^- strain. Since transposition of Tn10 was recA-independent, absence of the cointegrate molecule formed in a RecA^- strain strongly suggested that the cointegrate molecule is not an obligatory intermediate of transposition of Tn10.In the second method, mobilization of pACYC177 by R388 and by R388:: Tn10 was examined. The pACYC177 plasmid was mobilized by R388::Tn10 at a frequency of 10^-4 per donor but not by R388. It occurred, in most cases, by inverse transposition of R388::Tn10 to pACYC177 forming plasmids such as pACYC177::IS10-R388-IS10. Mobilization of pACYC177 by a Tn10-mediated cointegrate in the form of pACYC177::Tn10-R388-Tn10 was not observed in crosses using a Rec^- donor. These observations also suggested that transposition of Tn10 in Rec^- cells does not occur via the cointegrate molecule.     

Recombination between two TnA transposon sequences oriented as inverse repeats is found less frequently than between direct repeats

Summary Inverse repeats of the transposon Tn2660 in either a ColEl or an R6K replicon, with or without inversions of the parental DNA sequences between the repeats, show no detectable (<2%) evidence of recombination between the repeats after 60 generations of growth in either recA or recA ⁺ hosts. In contrast, attempts made to construct plasmids which carry two direct repeats by in vitro cleavage and ligation in a recA host were unsuccessful, although homologous plasmids with inverse repeats could be constructed, and other plasmids were found consistent with products of recombination between the direct repeats of a transient intermediate structure. It is concluded that in recA or recA ⁺ hosts recombination between direct repeats of a transposon is frequent, whereas recombination between inverse repeats of a homologous structure has not been observed. A model to explain this difference depends upon a mechanism that produces a nick in only one of the pair of strands at the internal resolution site (IRS) sequence of the transposon.     

Genetic manipulations in Rhizobium meliloti utilizing two new transposon Tn5 derivatives

Summary Two derivatives of the prokaryotic transposon Tn5 were constructed in vitro. In Tn5-233, the central area of Tn5, which carries resistance to kanamycin/neomycin, bleomycin and streptomycin, is replaced by a fragment carrying resistance to the aminocyclitol antibiotics gentamycin/kanamycin and streptomycin/spectinomycin. In Tn5-235, the Escherichia coli -galactosidase gene is inserted within the streptomycin resistance gene of Tn5, and constitutively expressed from a Tn5 promoter. Both constructs transpose with about the same frequency as Tn5 in Escherichia coli and Rhizobium meliloti. When a Tn5-derivative is introduced into an R. meliloti strain which already contains a different Tn5-derivative, in situ transposon replacement is obtained at high frequency, presumably by a pair of crossovers between the IS50 sequences at the ends of the incoming and resident transposons. In this way we converted a previously isolated recA::Tn5 mutant into the corresponding recA::Tn5-233 strain, which can now be used as a genetic background in the study of complementation of other Tn5-induced mutations. We also replaced the drug markers of several Tn5-induced exo mutants, which we were then able to map relative to each other by transduction with phage M12. In a strain carrying Tn5-235 located near Tn5-233, we were able to isolate deletions of the intervening markers, presumably resulting from general recombination between the two transposons, by screening for loss of the Lac⁺ phenotype. Unlike Tn5 itself, resident Tn5-233 does not appear to suppress transposition of another incoming Tn5-derivative.Abbreviations bp base pairs - Nm neomycin - Km kanamycin - Sm streptomycin - Sp spectinomycin - Gm gentamycin - Tc tetracycline - Tp trimethoprim - Ot oxytetracycline - Rf rifampicin - Xgal 5-bromo-4-chloro-3-indolyl--d-galactoside     

Generation of new transposons in vivo: an evolutionary role for the “staggered” head-to-head dimer and one-ended transposition

From a plasmid carrying the tnpA gene and one inverted repeat sequence (IR) of transposon Tn3, plasmids containing a structure characteristic of transposons, i.e., two IRs flanking a tnpA gene, were generated spontaneously in vivo. They appear to have arisen either through the formation of a “staggered” head-to-head dimer or by so-called one-ended transposition. These putative transposons could indeed transpose to, or form cointegrates with, a recipient plasmid. Based on these findings it is proposed that a primeval transposase gene and its target site evolved first, and subsequently gave rise to a “fully-fledged” transposon by head-to-head dimerization or one-ended transposition. Received: 30 October 1998 / Accepted: 1 April 1999     

Cointegration and resolution mediated by IS101 present in plasmid pSC101

Summary A certain class of cointegrate plasmids was found to occur between a pSC101 derivative and a second plasmid pBV320 in E. coli F^- cells. Cleavage analysis and DNA sequencing showed that the cointegrate plasmid contained direct repeats of an insertion sequence IS101 at the recombination junctions, indicating that formation of cointegrates was mediated by IS101, which is a natural constitutent of pSC101. These cointegrates were formed only in cells which contained the transposon gamma-delta, suggesting that the gamma-delta sequence, which provides transposase, is responsible for cointegration. Whenever the cointegrate plasmids were present in cells containing gamma-delta or its related transposon Tn3, the cointegrates were dissolved to give pBV320::IS101 due to recombination at duplicated IS101 sequences in the cointegrates, suggesting that both gamma-delta and Tn3, which provide a resolvase, are responsible for the resolution of the cointegrates. Comparison between the nucleotide sequence of IS101 and those of gamma-delta and Tn3 shows a high degree of homology in the regions that have been shown to be the binding sites of resolvases, as well as in the terminal inverted repeats. However, there is no homology between IS101 and the other element, gamma-delta or Tn3, in the internal resolution site, at which the resolution event may occur.Abbreviations Tc tetracycline - Cm chloramphenicol - Ap ampicillin - bp base pairs - kb kilobase pairs     

IS8- and Tn2353-mediated cointegration of the plasmids R15 and RP4::Tn1

Summary The plasmids R15 and RP4:: Tn1 form fused structures (85 Md and 92 Md cointegrates). The cointegrates do not resolve practically in recA ^– Escherichia coli cells and have a mean life-time of more than 50 generations in a recA ⁺ background.The 85 Md cointegrates were generated at a frequency of 4×10^–4 per R15 transconjugant during a mating between E. coli [R15; RP4:: Tn1] and E. coli [FColVBtrp:: Tn1755]. These plasmids carry two directly repeated copies of the mobile element IS8 at the junctions between R15 and RP4:: Tn1. The transposition of IS8 from RP4:: Tn1 to the R15 plasmid and the formation of hybrid molecules promoted by this process appear to be induced by the IS8 element of the Tn1755 structure during or after conjugal transfer of FColVBtrp:: Tn1755 into E. coli [R15; RP4:: Tn1] cells.The formation of the 92 Md cointegrates occurs at a frequency of 2×10^–5. The fused molecules of R15 and RP4:: Tn1 carry two direct copies of an 8.65 Md R15 fragment at the junctions between these replicons. The fragment has specific features of a new transposon. This element designated Tn2353 determines resistance to Hg, Sm and Su and contains two sites for each BamHI, BglII and SalI and three sites for both EcoRI and PstI. The physical map and some other characteristics of Tn2353 are presented.Abbreviations Ap ampicillin - EtBr ethidium bromide - Km kanamycin - Md megadaltons - Sm streptomycin - Su sulfanilamide - Tc tetracycline - [] brackets indicate plasmid-carrier state     

Transposition of a DNA fragment flanked by two inverted Tn1 sequences

The 32 Md fragment (derived from plasmid RP4::Tn1) carrying the Km^r gene and flanked by two inverted Tn1 elements is capable of recA-independent translocation to other plasmids. We designated this new transposon Tn1755. In various crosses, frequencies of Tn1755 transposition to plasmids Co1B-R3, R15 and F′ColVBtrp varied from 2.5 to 90% of the frequencies of Tn1 transposition. Tn1755 can integrate into various sites of the recipient plasmids. We failed to observe transposition of another RP4::Tn1 fragment flanked by two opposingly oriented Tn1 transposons and harboring the Tc^r gene. Presumably, to form a new transposable structure, other features must also be of importance.     

IS<Emphasis Type="Italic">Ppy1</Emphasis>, a novel mobile element of the ancient <Emphasis Type="Italic">Psychrobacter maritimus</Emphasis> permafrost strain: Translocations in <Emphasis Type="Italic">Escherichia coli</Emphasis> K-12 cells and formation of composite transposons

It was shown that IS element ISPpy1 isolated earlier in the permafrost strain Psychrobacter maritimus MR29-12 has a high level of functional activity in cells of the heterologous host Escherichia coli K-12. ISPpy1 can be translocated in E. coli cells by itself and mobilize adjacent genes and can also form composite transposons flanked by two copies of this element. Apart from translocations between different plasmids, the composite ISPpy1-containing transposon Tn5080a is capable of translocation from the plasmid into the E. coli chromosome with high frequency and from the chromosome into the plasmid. Among products of Tn5080a transposition into plasmid R388, simple insertions were predominantly formed together with cointegrates. Upon mobilization of adjacent genes with the use of one ISPpy1 copy, only cointegrates arise.     

细菌中介导多重耐药的Tn7转座子研究进展

转座子是介导细菌耐药性传播的重要可移动遗传元件。Tn7转座子与细菌耐药密切相关,其携带转座模块和Ⅱ类整合子系统。Tn7编码转座相关蛋白TnsABCDE进行“剪切-粘贴”机制转座,转座核心TnsABC也可与三链DNA或Cas-RNA复合物结合实现转座。近年来新发现了多种介导多重耐药的Tn7转座子,其在介导细菌抗生素、消毒剂和重金属抗性基因的获得、传播扩散等方面发挥了重要作用。本文综述了细菌中Tn7转座子的遗传结构、转座机制、流行以及新发现的介导多重耐药的Tn7转座子,以期为细菌中Tn7转座子的深入研究提供参考。     

Site-specific integration of genes into hot spots for recombination flanking aadA in Tn21 transposons.

Summary Tn21-related transposons are widespread among bacteria and carry various resistance determinants at preferential sites, hs1 and hs2. In an in vivo integrative recombination assay it was demonstrated that these hot spots direct the integration of aminoglycoside resistance genes like aadB from Klebsiella pneumoniae and aacAI from Serratia marcescens, in a recA ^– background. The maximum required recognition sequence which must be present in both the donor and recipient plasmids is 5 CTAAAACAAAGTTA 3 (hs2). The double-site-specific recombination occurred with a frequency of 10^–5–10^–6. The resulting structures include not only replicon fusion products but also more complex structures carrying two copies of the donor plasmid or simply the donor gene flanked by hs elements. hs1 and hs2 are thought to act as recognition sites for a trans-acting site-specific recombinase. By the use of Tn21 deletion derivatives, it has been shown that the recombinase is not encoded by Tn21. This new integrative recombination system is involved in the acquisition of new genes by Tn21-related transposons and their spread among bacterial populations.     

The transposable elements resident on the plasmids of Pseudomonas putida strain H, Tn 5501 and Tn 5502 , are cryptic transposons of the Tn 3 family

Genes for (methyl)phenol degradation in Pseudomonas putida strain H (phl genes) are located on the plasmid pPGH1. Adjacent to the phl catabolic operon we identified a cryptic transposon, Tn5501, of the Tn3 family (class II transposons). The genes encoding the resolvase and the transposase are transcribed in the same direction, as is common for the Tn501 subfamily. The enzymes encoded by Tn5501, however, show only the overall homology characteristic for resolvases/integrases and transposases of Tn3-type transposons. Therefore it is likely that Tn5501 is not a member of one of the previously defined subfamilies. Inactivation of the conditional lethal sacB gene was used to detect transposition of Tn5501. While screening for transposition events we found another transposon integrated into sacB in one of the sucrose-resistant survivors. This element, Tn5502, is a composite transposon consisting of Tn5501 and an additional DNA fragment. It is flanked by inverted repeats identical to those of Tn5501 and the additional fragment is separated from the Tn5501 portion by an internal repeat (identical to the left terminal repeat). Transposition of phenol degradation genes could not be detected. Analysis of sequence data revealed that the phl genes are not located on a Tn5501-like transposon. Received: 21 July 1997 / Accepted: 7 July 1998     

Genetic analysis of a transposon carrying toluene degrading genes on a TOL plasmid pWWO

Summary Toluene degrading (xyl) genes on a Pseudomonas TOL plasmid pWWO are located within a 39-kb DNA portion. The 56-kb region including these xyl genes and its 17-kb derivative with a deletion of the internal 39-kb portion transposed to various sites on target replicons such as pACYC184 and R388 in escherichia coli recA strains. Thus the 56- and 17-kb regions were designated Tn4651 and Tn4652, respectively. Genetic analysis of Tn4652 demonstrated that its transposition occurs by a two-step process, namely, cointegrate formation and its subsequent resolution. The presence in cis of DNA sequences of no more than 150 bp at both ends of Tn4652 was prerequisite for cointegrate formation, and this step was mediated by a trans-acting factor, transposase, which was encoded in a 3.0-kb segment at one end of the transposon. Cointegrate resolution took place site-specifically within a 200-bp fragment, which was situated 10 kb away from the transposase gene. Based on the stability of cointegrates formed by various mini-Tn4652 derivatives, it was shown that the cointergrate resolution requires two trans-acting factors encoded within 1.0- and 1.2-kb fragments that encompass the recombination site involved in the resolution.     

Construction of a recA mutant of Burkholderia (formerly Pseudomonas), cepacia

A recA mutant was constructed of a soil isolate of Burkholderia cepacia, strain ATCC 17616. Prior to mutagenesis, the recA gene was cloned from this strain by its ability to complement the methyl methanesulfonate sensitivity of an Escherichia coli recA mutant. Sequence analysis of the strain showed high sequence similarity (94% nucleic acid and 99% amino acid identity) with the recA gene previously cloned from a clinical isolate of B. cepacia, strain JN25. The subcloned recA gene from B. cepacia ATCC 17616 restored UV resistance and recombination proficiency to recA mutants of E. coli and Pseudomonas aeruginosa, as well as restoring the ability of D3 prophages to be induced to lytic growth from a RecA⁻ strain of P. aeruginosa. The recA mutant of B. cepacia ATCC 17616 was constructed by λ-mediated Tn5 mutagenesis of the cloned recA gene in E. coli, followed by replacement of the Tn5-interrupted gene for the wild-type allele in the chromosome of B. cepacia by marker exchange. The RecA⁻ phenotype of the mutant was demonstrated by the loss of UV resistance as compared to the parental strain. Southern hybridization analysis of chromosomal DNA from the mutant indicated the presence of Tn5 in the recA gene, and the location of the Tn5 insertion in the recA allele was identified by nucleotide sequence analysis. A test using the recA mutant to see if acquired resistance to d-serine toxicity in B. cepacia might be a result of RecA-mediated activities proved negative; nevertheless, RecA activity potentially contributes to the overall genomic plasticity of B. cepacia and a recA mutant will be useful in bioengineering of this species. Received: 24 January / Received revision: 11 July 1997 / Accepted: 25 August 1997     

A Tn21 terminal sequence within Tn501: complementation of tnpA gene function and transposon evolution

Summary The prokaryotic mercury-resistance transposon Tn501 contains a sequence, 80 nucleotides from one end, which is identical with an inverted terminal repeat (IR) of Tn21. This Tn21 IR sequence is used when Tn21 complements a TnpA^- derivative of Tn501, but not when Tn501 is used for the complementation. Complementation by Tn1721 shows a preference for the normal Tn501 IRs. The element (Tn820) transposed when Tn21 is used to complement a Hg^- TnpR^- TnpA^- Res^- deletion mutant of Tn501 contains the Tn21 IR sequence at one terminus and a Tn501 IR at the other. Transposition of Tn820 can be complemented by Tn501 and Tn1721, but at a much lower frequency than transposition of the parental element (Tn819) which has two Tn501 IRs. The relationship between the transposition functions of Tn501, Tn21 and Tn1721, and available nucleotide sequence data suggest that Tn501 evolved by the transposition of a Tn21-like element into another transposable element (similar to that found within Tn1721) followed by deletion of the Tn21-like transposition functions.Abbreviations used (IR) Inverted repeat - (Cb) carbenicillin - (Cm) chloramphenicol - (Sm) streptomycin - (Su) sulphonamide - (Tc) tetracycline - (Tp) trimethoprim     

Control of Tn7 transposition

Summary Our isolate of Tn7 (named Tn7S) contains an IS1 insertion, and this IS1 can be converted into Tn9. In vitro and in vivo deletions of Tn7S and Tn7S:: Tn9 define regions of the transposon required for antibiotic resistance and transposition. Complementation of deletion mutants by cloned Tn7 fragments indicates the existence of two regions, denoted tnp7A and tnp7B, required for all transposition events. Another region, denoted tnp7C, is required for transposition from the chromosome to RP1 but not for transposition from a small IncP-1 replicon to the chromosome. The presence of Tn7S terminal sequences in an RP1 replicon reduces the transposition of a second Tn7S derivative from the chromosome by about one order of magnitude. The measured frequency of Tn7S transpositions from a small IncP-1 replicon to the chromosome depends on the particular incompatibility system used to eliminate that replicon. Genetic and physical data indicate that high frequencies of Tn7S transposition to the chromosome (40%) are triggered by the IncP-1 incompatibility reaction, thus suggesting the existence of a Tn7 mechanism for sensing the state of the carrier replicon.     

Transposon mutagenesis in Azospirillum brasilense: isolation of auxotrophic and Nif- mutants and molecular cloning of the mutagenized nifDNA

Summary We report the successful mutagenesis of Azospirillum brasilense 29710 Rif Sm with transposon Tn5. The narrow host-range plasmid pGS9 (p15A replicon), which possesses broad host-range N-type transfer genes, was used as the suicide vehicle to deliver Tn5 in Azospirillum. Out of 900 colonies tested, 0.8% proved to be auxotrophic. One mutant altered in indoleacetic acid (auxin) biosynthesis was isolated and, in addition, three mutants completely defective in nitrogen fixation (nif) were obtained. All the mutants tested contained a single copy of Tn5 integrated randomly in the genome. The Tn5-mutagenized EcoRI fragments were cloned from the three Nif^- mutants. Physical analysis of cloned DNA showed that Tn5 was present on a different EcoRI fragment in each case, ranging in size from 15–17 kb. The nitrogenase structural genes (nifHDK) in A. brasilense 29710 Rif Sm were localized on a 6.7 kb EcoRI fragment. We found that Tn5 is not inserted in the nifHDK genes in the Nif^- mutants reported here. Site-directed mutagenesis using the cloned, Tn5-containing DNA from mutant Nif27(pMS188), produced a large number of Nif^- transconjugants of the A. brasilense 29710 Rif wild-type strain, showing the linkage between Tn5 insertion and the Nif^- phenotype. This is the first time that transposon-mutagenized auxotrophic, Nif^- and other mutants have been available for genetic analysis in Azospirillum. This should greatly facilitate the cloning and mapping of genes involved in nitrogen fixation as well as in many other phenotypic characteristics of Azospirillum.     

The transposable elements resident on the plasmids of Pseudomonas putida strain H, Tn5501 and Tn5502, are cryptic transposons of the Tn3 family

Genes for (methyl)phenol degradation in Pseudomonas putida strain H (phl genes) are located on the plasmid pPGH1. Adjacent to the phl catabolic operon we identified a cryptic transposon, Tn5501, of the Tn3 family (class II transposons). The genes encoding the resolvase and the transposase are transcribed in the same direction, as is common for the Tn501 subfamily. The enzymes encoded by Tn5501, however, show only the overall homology characteristic for resolvases/integrases and transposases of Tn3-type transposons. Therefore it is likely that Tn5501 is not a member of one of the previously defined subfamilies. Inactivation of the conditional lethal sacB gene was used to detect transposition of Tn5501. While screening for transposition events we found another transposon integrated into sacB in one of the sucrose-resistant survivors. This element, Tn5502, is a composite transposon consisting of Tn5501 and an additional DNA fragment. It is flanked by inverted repeats identical to those of Tn5501 and the additional fragment is separated from the Tn5501 portion by an internal repeat (identical to the left terminal repeat). Transposition of phenol degradation genes could not be detected. Analysis of sequence data revealed that the phl genes are not located on a Tn5501-like transposon.     

Vectors for transposon mutagenesis of non-enteric bacteria

Summary We have constructed a series of transposon delivery vectors derived from pRK2013. Since pRK2013 has a broad host range transfer system and a ColE1 replicon, it can be transferred to, but not replicated in, many nonenteric gram-negative bacteria. Thus pRK2013 provides an effective mechanism for the transient introduction of a transposon. Delivery vectors containing Tn7 (tmp str), Tn10 (tet), Tn10 HH104 (tet), or Tn5-132 (tet) have been constructed. When transposition in Caulobacter crescentus was examined, both Tn7 and Tn5-132 were found to transpose efficiently. In contrast, although the antibiotic resistances of Tn10 and Tn501 (mer) were expressed in C. crescentus, no transposition was observed with either transposon. However, transposition of Tn10 from the Tn10 vectors did occur in Acinetobacter calcoaceticus, and transposition of Tn501 from pMD100 has been demonstrated in Rhizobium japonicum (Bullerjahn and Benzinger 1984). Thus, transposon-host interactions play an important role in the determination of whether a particular transposon can transpose in a given host. Futhermore, the results with C. crescentus indicate that there must be different requirements for host interactions for Tn10 and Tn501 than for Tn5 and Tn7.     

The new class 11 transposon Tn163 is plasmid-borne in two unrelated Rhizobium leguminosarum biovar viciae strains

Tn163 is a transposable element identified in Rhizobium leguminosarum bv. viciae by its high insertion rate into positive selection vectors. The 4.6 kb element was found in only one further R. leguminosarum bv. viciae strain out of 70 strains investigated. Both unrelated R. leguminosarum bv. viciae strains contained one copy of the transposable element, which was localized in plasmids native to these strains. DNA sequence analysis revealed three large open reading frames (ORFs) and 38 bp terminal inverted repeats. ORF1 encodes a putative protein of 990 amino acids displaying strong homologies to transposases of class 11 transposons. ORF2, transcribed in the opposite direction, codes for a protein of 213 amino acids which is highly homologous to DNA invertases and resolvases of class II transposons. Homology of ORF1 and ORF2 and the genetic structure of the element indicate that Tn163 can be classified as a class II transposon. It is the first example of a native transposon in the genus Rhizobium. ORF3, which was found not to be involved in the transposition process, encodes a putative protein (256 amino acids) of unknown function. During transposition Tn163 produced direct repeats of 5 bp, which is typical for transposons of the Tn3 family. However, one out of the ten insertion sites sequenced showed a 6 by duplication of the target DNA; all duplicated sequences were A/T rich. Insertion of Tn163 into the sacB gene revealed two hot spots. Chromosomes of different R. leguminosarum bv. viciae strains were found to be highly refractory to the insertion of Tn163.