Analysis of Tn3 sequences required for transposition and immunity

The Tn3-family of Replicative Transposons

Transposons of the Tn3 family form a widespread and remarkably homogeneous group of bacterial transposable elements in terms of transposition functions and an extremely versatile system for mediating gene reassortment and genomic plasticity owing to their modular organization. They have made major contributions to antimicrobial drug resistance dissemination or to endowing environmental bacteria with novel catabolic capacities. Here, we discuss the dynamic aspects inherent to the diversity and mosaic structure of Tn3-family transposons and their derivatives. We also provide an overview of current knowledge of the replicative transposition mechanism of the family, emphasizing most recent work aimed at understanding this mechanism at the biochemical level. Previous and recent data are put in perspective with those obtained for other transposable elements to build up a tentative model linking the activities of the Tn3-family transposase protein with the cellular process of DNA replication, suggesting new lines for further investigation. Finally, we summarize our current view of the DNA site-specific recombination mechanisms responsible for converting replicative transposition intermediates into final products, comparing paradigm systems using a serine recombinase with more recently characterized systems that use a tyrosine recombinase.

Does the upstream region possessing MULE-like sequence in rice upregulate PsbS1 gene expression?

The genomic nucleotide sequences of japonica rice (Sasanishiki and Nipponbare) contained about 2.7-kb unique region at the point of 0.4-kb upstream of the OsPsbS1 gene. In this study, we found that japonica rice with a few exceptions possessing such DNA sequences [denoted to OsMULE-japonica specific sequence (JSS)] is distinct by the presence of Mutator-like-element (MULE). Such sequence was absent in most of indica cultivars and Oryza glaberrima. In OsMULE-JSS1, we noted the presence of possible target site duplication (TSD; CTTTTCCAG) and about 80-bp terminal inverted repeat (TIR) near TSD. We also found the enhancement ofOsPsbS1 mRNA accumulation by intensified light, which was not associated with the DNA methylation status in OsMULE/JSS. In addition, O. rufipogon, possible ancestor of modern rice cultivars was found to compose PsbS gene of either japonica (minor) or indica (major) type. Transient gene expression assay showed that the japonica type promoter elevated a reporter gene activity than indica type.

Mutator-like elements with multiple long terminal inverted repeats in plants

Mutator-like transposable elements (MULEs) are widespread in plants and the majority have long terminal inverted repeats (TIRs), which distinguish them from other DNA transposons. It is known that the long TIRs of Mutator elements harbor transposase binding sites and promoters for transcription, indicating that the TIR sequence is critical for transposition and for expression of sequences between the TIRs. Here, we report the presence of MULEs with multiple TIRs mostly located in tandem. These elements are detected in the genomes of maize, tomato, rice, and Arabidopsis. Some of these elements are present in multiple copies, suggesting their mobility. For those elements that have amplified, sequence conservation was observed for both of the tandem TIRs. For one MULE family carrying a gene fragment, the elements with tandem TIRs are more prevalent than their counterparts with a single TIR. The successful amplification of this particular MULE demonstrates that MULEs with tandem TIRs are functional in both transposition and duplication of gene sequences.

Different versions of Pseudomonas syringae pv. tomato DC3000 exist due to the activity of an effector transposon

SUMMARY The plant pathogenic bacterium Pseudomonas syringae pv. tomato strain DC3000 is a key model organism to study plant-pathogen interactions. We realized that two versions of this strain, which carry plasmids of different sizes, exist in our strain collections. The difference was located to a 9.4-kb deletion within the larger of the two endogenous plasmids encompassing the partitioning genes parA and parB and a putative mobile element encoding the type III effector hopAM1-2 (formerly avrPpiB2). Both plasmid variants are lost in similar frequency, indicating that the partitioning genes are not essential for stability of the plasmid. In addition, the deletion derivative strain DC3001 exhibited the same virulence towards Arabidopsis as strain DC3000. The deletion site in DC3001 is located immediately adjacent to a putative transposon that carries the effector hopX1 (formerly avrPphE), suggesting that the deletion originated from an aberrant transposition event of this element. By tagging the hopX1 transposon with an antibiotic resistance cassette on a suicide plasmid it was shown that the element is functional and produces a target site duplication of 5 bp. The plasmid also integrated into the chromosome in several cases, possibly mediated by one-ended transposition of the hopX1 transposon. This is the first report that describes an active effector-transposon. Comparison of DC3000 strains from several sources revealed that strains exist with differences in the endogenous plasmid composition.

Large-scale mutagenesis of the yeast genome using a Tn7-derived multipurpose transposon

We present here an unbiased and extremely versatile insertional library of yeast genomic DNA generated by in vitro mutagenesis with a multipurpose element derived from the bacterial transposon Tn7. This mini-Tn7 element has been engineered such that a single insertion can be used to generate a lacZ fusion, gene disruption, and epitope-tagged gene product. Using this transposon, we generated a plasmid-based library of approximately 300,000 mutant alleles; by high-throughput screening in yeast, we identified and sequenced 9032 insertions affecting 2613 genes (45% of the genome). From analysis of 7176 insertions, we found little bias in Tn7 target-site selection in vitro. In contrast, we also sequenced 10,174 Tn3 insertions and found a markedly stronger preference for an AT-rich 5-base pair target sequence. We further screened 1327 insertion alleles in yeast for hypersensitivity to the chemotherapeutic cisplatin. Fifty-one genes were identified, including four functionally uncharacterized genes and 25 genes involved in DNA repair, replication, transcription, and chromatin structure. In total, the collection reported here constitutes the largest plasmid-based set of sequenced yeast mutant alleles to date and, as such, should be singularly useful for gene and genome-wide functional analysis.

A shuttle mutagenesis system for tagging genes in the yeast Yarrowia lipolytica

A shuttle mutagenesis system was developed for the dimorphic yeast Yarrowia lipolytica. This system combines transposon insertions generated in Escherichia coli with the transformation of yeast with the Tn-mutagenized DNA. The mini-transposon mTn-3xHA/GFP, used in Saccharomyces cerevisiae for producing stable insertions, was adapted for use in the yeast Y. lipolytica. The mTnYl1 transposon (for mini-Tn of Y. lipolytica) confers resistance to tetracycline in E. coli. It also contains the Y. lipolytica URA3 gene for selection of yeast transformants, and the coding sequence for the S65T mutant form of GFP. The rare cutter endonuclease, I-SceI, restriction site, which enables identification of the chromosomal localization of mutagenized genes, was also incorporated. mTnYl1 was first tested on the ACO1 gene, which encodes an Acyl CoA oxidase isozyme. The mutagenesis system was further validated on a Y. lipolytica genomic DNA library constructed in a pHSS6 derivative vector. Mutants with a particular morphology or defective for alkane, fatty acids and oil degradation were obtained.

UV photoaffinity labeling of Tn3 transposase--DNA complexes: identification of DNA binding domains

The prokaryotic transposon Tn3 requires the transposase protein, as well as the cis-acting terminal inverted repeats (IRs), for transposition. The first step in the transposition process requires transposase binding to the IRs, as well as target site selection for element insertion. The primary aim of this study is to define the relationship between the structure of Tn3 transposase and its DNA binding functions. We have defined, by UV cross-linking, two broad regions of transposase that interact with DNA: a 70-kDa N-terminal domain and a 30-kDa C-terminal domain. The 70-kDa N-terminal domain encompasses the IR sequence specific binding domain, as well as a nonspecific DNA binding domain that has been previously described. We have also defined, by UV cross-linking, a region in the nonspecific DNA binding domain centered at amino acids 376 and 381 that is in contact with DNA. We have used site-directed mutagenesis of amino acids 376 and 381 to help delineate the function of this region of the transposase protein. Mutations in this region reduce transposition frequency to 30-40% of the wild type. These mutations reduce nonspecific DNA binding three- to four-fold but do not appear to affect specific binding to the IR. Transposition immunity is unaffected by mutations in the nonspecific DNA binding domain. This suggests that this region may be involved in target site selection.

Insertion site specificity of the transposon Tn3

The Tn3-deletion method [Davies and Hutchison, Nucleic Acids Res. 19, 5731-5738, (1991)] was used to sequence a 9.4 kb DNA fragment. Transpositional 'warm' spots were not a limiting factor but a 935 bp 'cold' spot was completed using a synthetic oligonucleotide primer. Two hundred and twenty three miniTn3 insertion sites from three sequencing projects were aligned and a 19 bp asymmetric consensus site was identified. There is no absolute sequence requirement at any position in this consensus, so insertion occurs promiscuously (approximately 37% of sites are potential targets). In our sequencing projects, multiply targeted sites always closely matched the consensus, although not all close matches were targeted frequently. The 935 bp cold spot showed no unusual features when analysed with the consensus sequence. The consensus can be used to accurately predict likely insertion sites in a new sequence. Synthetic oligonucleotides based on the consensus and a known hot spot for Tn3 were mutagenised. These sequences were not hot spots in our vectors, suggesting that the primary sequence alone is not sufficient to create an insertional hot spot. We conclude that some other factor, such as DNA secondary structure, also plays an important role in target site selection for the transposon Tn3.

Conversion of pBR322-based plasmids into broad-host-range vectors by using the Tn3 transposition mechanism

We constructed a series of transposon vectors which allow efficient in vitro gene manipulation and subsequent introduction of cloned DNA into a variety of gram-negative bacteria. Transfer of the cloned fragment from these multicopy plasmids into self-transmissible broad-host-range vectors is achieved in vivo, using the Tn3 transposition mechanism. Transposition into a variety of broad-host-range plasmids proceeds efficiently, and the resulting recombinant plasmids can be readily transferred and maintained in a variety of gram-negative bacteria. The utility of the transposable vectors was demonstrated by the introduction and expression of the lacIPOZY sequences of Escherichia coli into Pseudomonas putida strains, allowing them to utilize lactose as a sole source of carbon and energy.

Conduction of pEC22, a plasmid coding for MR.EcoT22I, mediated by a resident Tn3-like transposon, Tn5396

pEC22 is a small plasmid that encodes the restriction-modification system MR.EcoT22I. Restriction and functional analysis of the plasmid identified the positions of genes encoding that system. The plasmid is able to be conducted by conjugal plasmids, a process mediated by a transposon contained within pEC22. This cryptic transposon, called Tn5396, was isolated from pEC22 and partially sequenced. The sequence of Tn5396 is for the most part typical of transposons of the Tn3 family and is most similar to that of Tn1000. The transposon differs from closely related transposons in that it lacks well-conserved sequences in the inverted-repeat region and has an unusually long terminal inverted repeat. Consideration of regions of internal sequence similarity in this and other transposons in the Tn3 family supports a theory of the mechanism by which the ends of Tn3-like transposons may maintain substantial identity between their inverted repeats over the course of evolutionary time.

Functional analysis of the two domains in the terminal inverted repeat sequence required for transposition of Tn3

Bacterial transposon Tn3 has a 38-bp terminal inverted repeat (IR) sequence. The IR sequence has been divided into two domains, A and B, of which domain B is bound by transposase, and domain A is not Here, we defined the two domains more precisely by constructing three IR mutants with a 2-bp substitution at relevant sites within the IR sequence, followed by examination of the binding of transposase to the fragments containing these IR mutants: domain A was located at bp 1-11, whereas domain B was at bp 12-38. To see if the two domains in the IR are functionally distinct, we constructed mini-Tn3 derivatives flanked by two IRs with various 2-bp substitutions within domain A or B, and analyzed their ability to mediate cointegration. The mini-Tn3 derivatives flanked by IR(A+ B+) and IR(A- B+) [or IR(A+ B-)] and those flanked by IR(A-B+) and IR(A+ B-) mediate cointegration more efficiently than the mini-Tn3 derivatives flanked by two IR(A- B+)s or by two IR(A+ B-)s. These results and others presented here indicate that the two domains of IR are functionally distinct in promoting cointegration.

Transposition in prokaryotes: transposon Tn501

Bacteria contain a large number of transposable elements that can be categorized in four major groups according to their mechanisms of transposition. These are: class I: insertion sequences (IS) and compound transposons (with IS sequences at their termini) which usually require only one protein for transposition to occur (e.g. Tn10); class II: complex transposons and insertion sequences with short inverted repeats in which transposition is replicative and requires two gene products (e.g. Tn3); class III: transposable bacteriophage (e.g. Mu). The fourth group consists of the transposons and IS of variable mechanism, which do not fall into the above classes (e.g. Tn7). We have studied the mechanism of transposition of Tn501 and Tn21, closely-related class II mercury-resistance transposons, which transpose via a cointegrate intermediate. By using genetic methods, we have shown that the region of the 989 amino acid transposase between amino acids 57 and 186 determines the specificity for recognition of the 38-bp terminal inverted repeats of the transposon in normal transposition and for replicon fusion catalysed by a single transposon terminus. The Tn501 transposase has been over-expressed and is functional in vivo, raising the frequency of transposition approximately 10(4)-fold.

A Tn3 derivative that can be used to make short in-frame insertions within genes

A Tn3 derivative was constructed to make small in-frame insertions within genes. The transposon contains the URA3 gene, the tetA gene, a truncated lacZ, and phage P1 loxP recombination sites at either end. Insertions that have fused lacZ to an open reading frame are lac+ because they express the truncated lacZ. In the presence of the phage P1 cyclization recombinase cre, the transposon can delete the URA3, tetA, and lacZ genes between the two loxP sites. The remaining short imperfect palindrome contains the ends of Tn3 and a loxP site and does not contain a translational termination codon in the correct reading frame. We have analyzed several insertions within the yeast HO gene. Several insertions inactivate HO and prohibit initiation of mating-type switching. In contrast, an epitope inserted in the central portion encodes a functional HO endonuclease.

Mutations in the inverted repeats of Tn3 affect binding of transposase and transposition immunity

In order to better understand the interaction between the inverted repeats (IRs) of the transposon Tn3 and Tn3 transposase, we have looked at the effects of mutations within the IRs on binding of transposase and transposition immunity. Binding of transposase to mutated IRs was measured using a site-specific nitrocellulose filter binding assay and by DNase I protection studies. Transposition immunity was measured in vivo using a transposition mating-out assay. The most important determinants for binding of transposase are present within the inside 21 base-pairs of the IR and several single base-pair mutations significantly reduce binding. Base-pair mutations which do not effect binding have strong negative effects on transposition immunity indicating that simple binding of transposase to the IR is not sufficient for the establishment of transposition immunity.

Mutational analysis of the inverted repeats of Tn3

The transposase protein and the terminal inverted repeat sequences of the prokaryotic transposon Tn3 are essential for transposition. In order to determine the sequences within the inverted repeat necessary for transposition and interaction with transposase, we have constructed a series of mini-Tn3s in which specific mutations have been introduced into the inverted repeats. The effects of these mutations on transposition have been assayed in vivo using a mating-out transposition assay. Several single base-pair mutations within the transposase binding site reduce transposition frequency. Mutations that affect transposition show a greater effect when present in both inverted repeats than when present in only one inverted repeat.

Tn3 transposition immunity is conferred by the transposase-binding domain in the terminal inverted-repeat sequence of Tn3

A series of mutant terminal inverted repeats (IRs), having 2 bp substitutions at various sites within the 38-bp IR sequence of the ampicillin-resistance transposon Tn3, were tested for transposition immunity to Tn3. Mutations within region 1-10 in the IR did not affect transposition immunity, while mutations within region 13-38 inactivated the immunity function. These two regions corresponded to domain A which was not bound specifically by Tn3 transposase and to domain B which was bound by the transposase, respectively. This indicates that specific binding of transposase to domain B within the IR sequence is responsible for transposition immunity.

DNA sequence analysis of five genes; tnsA, B, C, D and E, required for Tn7 transposition

A region of DNA sequence of the bacterial transposon Tn7, which is required for transposition, has been determined. This DNA sequence completes an 8351 base pair (bp) region containing five long open reading frames (ORF's) that correspond to the genetically defined genes, tnsA, B, C, D and E, required for Tn7 transposition. All of the ORF's are oriented in the same direction, ie. inward from the element's right end. The genes are in a very compact arrangement with the presumed initiation codons never more than two bases beyond the preceding termination codon. Domains with similarity to the helix-turn-helix genre of Cro-like, sequence specific DNA binding sites occur within the deduced amino acid (a.a.) sequence of the TnsA, TnsB, TnsD and TnsE proteins. Translation of the tnsC ORF reveals strong homology to a consensus sequence for nucleotide binding sites as well as a region of similarity to a transcriptional activator (MalT). No striking a.a. sequence similarity to other DNA recombinases is observed. The possible roles of these proteins in Tn7 transposition is discussed in light of the analysis presented.

Two domains in the terminal inverted-repeat sequence of transposon Tn3

Tn3 and related transposons have terminal inverted repeats (IR) of about 38 bp that are needed as sites for transposition. We made mini-Tn3 derivatives which had a wild-type IR of Tn3 at one end and either the divergent IR of the Tn3-related transposon, gamma delta or IS101, or a mutant IR of Tn3 at the other end. We then examined both in vivo transposition (cointegration between transposition donor and target molecules) of these mini-Tn3 elements and in vitro binding of Tn3-encoded transposase to their IRs. None of the elements with an IR of gamma delta or IS101 mediated cointegration efficiently. This was due to inefficient binding of transposase to these IR. Most mutant IR also interfered with cointegration, even though transposase bound to some mutant IR as efficiently as it did to wild type. This permitted the Tn3 IR sequence to be divided into two domains, named A and B, with respect to transposase binding. Domain B, at positions 13-38, was involved in transposase binding, whereas domain A, at positions 1-10, was not. The A domain may contain the sequence recognized by some other (e.g., host) factor(s) to precede the actual cointegration event.

Effects of variation of inverted-repeat sequences on reactions mediated by the transposase of Tn21

The frequencies of one-ended transposition and normal transposition of derivatives of Tn21 that contain mutant inverted-repeat sequences (IRs) have been measured. In general, there was a linear relationship between the log of the frequency of one-ended transposition of a mutant IR and the log of the frequency of normal transposition of an element flanked by a wild-type IR at one end and by the mutant IR at the other. This implied that one-ended and normal transposition share the rate-limiting step that determines the frequency of transposition and that both IRs are involved in the rate-limiting step in normal transposition. Surprisingly, it was found that only the outer 18 base pairs of the IR of Tn21 engaged accurately in both one-ended and normal transposition, at about 1% of the frequency of the wild-type IR.

Transposon Tn7. cis-Acting sequences in transposition and transposition immunity

We have identified and characterized the cis-acting sequences at the termini of the bacterial transposon Tn7 that are necessary for its transposition. Tn7 participates in two kinds of transposition event: high-frequency transposition to a specific target site (attTn7) and low-frequency transposition to apparently random target sites. Our analyses suggest that the same sequences at the Tn7 ends are required for both transposition events. These sequences differ in length and nucleotide structure: about 150 base-pairs at the left end (Tn7L) and about 70 base-pairs at the right end (Tn7R) are necessary for efficient transposition. We also show that the ends of Tn7 are functionally distinct: a miniTn7 element containing two Tn7R ends is active in transposition but an element containing two Tn7L ends is not. We also report that the presence of Tn7's cis-acting transposition sequences anywhere in a target replicon inhibits subsequent insertion of another copy of Tn7 into either an attTn7 target site or into random target sites. The inhibition to an attTn7 target site is most pronounced when the Tn7 ends are immediately adjacent to attTn7. We also show that the presence of Tn7R's cis-acting transposition sequences in a target replicon is necessary and sufficient to inhibit subsequent Tn7 insertion into the target replicon.

Nucleotide sequences required for Tn3 transposition immunity

The Tn3 transposon inserts at a reduced frequency into a plasmid already containing a copy of Tn3, a phenomenon known as transposition immunity. The cis-acting site on Tn3 responsible for immunity was mapped by deletions from each side to be within the terminal 38-base-pair sequence that is inversely repeated at the ends of Tn3. Two palindromic sequences are present in the essential part of this region. Some deletions conferred only partial immunity, and others conferred negative immunity. Multiple copies of partially immune ends conferred additional immunity. No other part of Tn3 was necessary for immunity.

IS257 from Staphylococcus aureus: member of an insertion sequence superfamily prevalent among gram-positive and gram-negative bacteria

The nucleotide sequences for the IS257 family of insertion sequences from Staphylococcus aureus were compared with those of the ISS1 family from Streptococcus lactis and the IS15 family which is widespread amongst Gram-negative bacteria. These elements have a striking degree of similarity in both their putative transposase polypeptide sequences and their nucleotide sequences (40 to 64% between pairs), including 12 out of 14 bp conservation in their terminal inverted repeats. The evolutionary distance between the IS15 family and the IS257 and ISS1 families of Gram-positive origin is approximately twice that between the IS257 and ISS1 families. Analysis of base substitutions in the three sequences has provided insights into the effect of selection for the G + C content of immigrant genes to conform to that of their hosts, and into the evolution of biases in overall amino acid composition of cellular proteins in prokaryotes and eukaryotes. The IS257, ISS1, IS15 families form a superfamily of insertion sequences that has been involved in the spread of a number of antimicrobial resistance determinants in Gram-positive and Gram-negative pathogens.

Outreading promoters are located at both ends of the gamma-delta transposon

Two plasmids were isolated containing oppositely oriented gamma-delta insertions between the wild-type transcription initiation site of the nifHDKY operon and the nifH coding sequence. The nifHDKY promoter of Klebsiella pneumoniae, similar to other nitrogen fixation (nif) promoters, normally requires the products of ntrA and nifA for activity. Mutations that allowed constitutive expression of the nifHDKY operon were searched for by transforming a plasmid, containing the regulatory region of this operon followed by an in-frame nifH'-'lacZ fusion, into a Lac- Escherichia coli strain (which contains no nifA) and screening for Lac+ derivatives. The plasmids described here were isolated from such derivatives and directed the constitutive expression of beta-galactosidase. Deletion analysis indicated that gamma-delta promoters other than those transcribing tnpA and tnpR were involved in this expression. Nuclease S1 mapping revealed outward-reading transcription initiation sites in both the gamma end and the delta end of the transposon. Most interestingly, one initiation site on each end was located in corresponding positions within the terminal inverted repeats. The sites were in the center of the longest sequence, of 12 bp, contiguously conserved between the terminal inverted repeats of gamma-delta and the related transposon Tn3. In gamma-delta and Tn3, this sequence has been recently implicated in transposase binding. These results suggest a possible interrelationship between transcription from the "end" promoters and transposition.

Binding of the Tn3 transposase to the inverted repeats of Tn3

The transposase protein and the inverted repeat sequences of Tn3 are both essential for Tn3 cointegrate formation and transposition. We have developed two assays to detect site-specific binding of transposase to the inverted repeats: (1) a nitrocellulose filter binding assay in which transposase preferentially retains DNA fragments containing inverted repeat sequences, and (2) a DNase 1 protection assay in which transposase prevents digestion of the inverted repeats by DNase 1. Both assays show that transposase binds directly to linear, duplex DNA containing the inverted repeats. The right inverted repeat of Tn3 binds slightly more strongly than the left one. Site-specific binding requires magnesium but does not require a high energy cofactor.

Tn3-like elements: molecular structure, evolution

The structure and transposition mechanism of Tn3-elements are described. Different studies showed that Tn21, Tn501, Tn1721 and Tn3926 are closely related. An evolution model for these transposons is proposed.

Specific binding of transposase to terminal inverted repeats of transposable element Tn3

Tn3 transposase, which is required for transposition of Tn3, has been purified by a low-ionic-strength-precipitation method. Using a nitrocellulose filter binding assay, we have shown that transposase binds to any restriction fragment. However, binding of the transposase to specific fragments containing the terminal inverted repeat sequences of Tn3 can be demonstrated by treatment of transposase-DNA complexes with heparin, which effectively removes the transposase bound to the other nonspecific fragments at pH 5-6. DNase I "footprinting" analysis showed that the transposase protects an inner 25-base-pair region of the 38-base-pair terminal inverted repeat sequence of Tn3. This protection is not dependent on pH. Interestingly, binding of the transposase to the inverted repeat sequences facilitates DNase I to nick at the end of the Tn3 sequence. It was also observed that the transposase protects the end regions of restriction fragments with a cohesive sequence at their 5' end or with a flush end from DNase I cleavage. The specific and nonspecific binding of transposase to DNA is ATP-independent.

Transposition of Tn1000: in vivo properties

Transposition mediated by the Tn1000 transposase was investigated by using transposon variants carrying synthetic or wild-type termini but no intact Tn1000 genes. Transposon Tn1001, whose only homologies to Tn1000 are in its 38-base-pair terminal inverted repeats, transposed at the same rate as Tn1005, an artificial construct carrying wild-type Tn1000 termini and approximately 1 kilobase of flanking Tn1000 DNA at each end, when transposase was supplied in trans. The majority of the transpositions into pOX38 gave rise to cointegrates, but approximately 10% of the products expressed phenotypes of direct transpositions. The expression and temperature dependence of the tnpA gene product were examined by studying transposition of Tn1001 to bacteriophage lambda. The temperature optimum for transposition was 37 degrees C, and the transposase was stable for up to 2 h at this temperature.

Transposition of Tn1000: activity of single or directly repeated termini

Tn1000 (gamma delta) termini IRR and IRL, or direct repetitions of IRR-IRL carried by pBR322 derivatives mediate cointegration with pOX38 at similar rates. Structures of product plasmids indicate that the transposed segments correspond to DNA bounded by IR segments in the donor plasmid. Such structures could arise by symmetric transposition from a replication intermediate.

Mini-Mu transduction: cis-inhibition of the insertion of Mud transposons

Mud (mini-Mu) transposons are defective phage Mu genomes that conserve the Mu ends. The transduction of Mud transposons is strictly dependent on Mu complementation, inefficient, and affected by modifications in the Mud internal sequences. The transduction of Mud transposons depends on transposition, which appears to be low, relative to wild-type Mu. Insertions of Mud into a plasmid can be frequently recovered among transductants; new Mud insertions into plasmids that already have both Mu ends, or just one, are rarely found. This suggests that the presence of Mu ends "immunizes" the plasmid against further insertion. This phenomenon may be similar to the transposition immunity of Tn3.

Identification of the DNA sequence required for transposition immunity of the gamma delta sequence

A plasmid containing the transposon gamma delta sequence was immune to further insertion of gamma delta (transposition immunity). Plasmids carrying a fragment containing either 0.2 kilobase pairs of the gamma end or 0.4 kilobase pairs of the delta end of the gamma delta sequence were immune, and other parts of the gamma delta sequence did not confer immunity. The terminal 38-base-pair (bp) sequence of the delta end of the gamma delta was sufficient to confer immunity, the 38-bp sequence of the gamma end conferred only moderate immunity, and the terminal 35-bp sequence, which was completely identical at both the gamma and delta ends, was insufficient to confer immunity.

Characterization of the terminal sequences flanking the transposon that carries the Escherichia coli enterotoxin STII gene

The Escherichia coli enterotoxin STII gene is flanked by two repeat sequences, approx. 600 bp each and 8 kb apart. This 9-kb DNA fragment has been shown to transpose as a unit and is thus considered a transposon. It is presently designated as Tn4521. In this study, the two terminal sequences of Tn4521 cloned in pPS1 were localized, isolated, and characterized. The two terminal sequences were found to be composed of IS2 sequences and were in an inverted repeat orientation. However, neither repeat contained a complete IS2. The LTR contained bp 1-722, whereas the RTR contained bp 17-536 and 969-1327, all three of the IS2 sequence.