Functional organization of the ends of IS1: specific binding site for an IS1-encoded protein

Protein-DNA interactions define the mechanistic aspects of circle formation and insertion reactions in IS2 transposition

BACKGROUND

Transposition in IS3, IS30, IS21 and IS256 insertion sequence (IS) families utilizes an unconventional two-step pathway. A figure-of-eight intermediate in Step I, from asymmetric single-strand cleavage and joining reactions, is converted into a double-stranded minicircle whose junction (the abutted left and right ends) is the substrate for symmetrical transesterification attacks on target DNA in Step II, suggesting intrinsically different synaptic complexes (SC) for each step. Transposases of these ISs bind poorly to cognate DNA and comparative biophysical analyses of SC I and SC II have proven elusive. We have prepared a native, soluble, active, GFP-tagged fusion derivative of the IS2 transposase that creates fully formed complexes with single-end and minicircle junction (MCJ) substrates and used these successfully in hydroxyl radical footprinting experiments.

RESULTS

In IS2, Step I reactions are physically and chemically asymmetric; the left imperfect, inverted repeat (IRL), the exclusive recipient end, lacks donor function. In SC I, different protection patterns of the cleavage domains (CDs) of the right imperfect inverted repeat (IRR; extensive in cis) and IRL (selective in trans) at the single active cognate IRR catalytic center (CC) are related to their donor and recipient functions. In SC II, extensive binding of the IRL CD in trans and of the abutted IRR CD in cis at this CC represents the first phase of the complex. An MCJ substrate precleaved at the 3' end of IRR revealed a temporary transition state with the IRL CD disengaged from the protein. We propose that in SC II, sequential 3' cleavages at the bound abutted CDs trigger a conformational change, allowing the IRL CD to complex to its cognate CC, producing the second phase. Corroborating data from enhanced residues and curvature propensity plots suggest that CD to CD interactions in SC I and SC II require IRL to assume a bent structure, to facilitate binding in trans.

CONCLUSIONS

Different transpososomes are assembled in each step of the IS2 transposition pathway. Recipient versus donor end functions of the IRL CD in SC I and SC II and the conformational change in SC II that produces the phase needed for symmetrical IRL and IRR donor attacks on target DNA highlight the differences.

Transposable elements and their identification

Most genomes are populated by thousands of sequences that originated from mobile elements. On the one hand, these sequences present a real challenge in the process of genome analysis and annotation. On the other hand, there are very interesting biological subjects involved in many cellular processes. Here, we present an overview of transposable elements (TEs) biodiversity and their impact on genomic evolution. Finally, we discuss different approaches to the TEs detection and analyses.

IS6110, a Mycobacterium tuberculosis complex-specific insertion sequence, is also present in the genome of Mycobacterium smegmatis, suggestive of lateral gene transfer among mycobacterial species

IS6110 is an insertion element found exclusively within the members of the Mycobacterium tuberculosis complex (MTBC), and because of this exclusivity, it has become an important diagnostic tool in the identification of MTBC species. The restriction of IS6110 to the MTBC is hypothesized to arise from the inability of these bacteria to exchange DNA. We have identified an IS6110-related element in a strain of Mycobacterium smegmatis. The presence of IS6110 indicates that lateral gene transfer has occurred among mycobacterial species, suggesting that the mycobacterial gene pool is larger than previously suspected.

Causes of insertion sequences abundance in prokaryotic genomes

Insertion sequences (ISs) are the smallest and most frequent transposable elements in prokaryotes where they play an important evolutionary role by promoting gene inactivation and genome plasticity. Their genomic abundance varies by several orders of magnitude for reasons largely unknown and widely speculated. The current availability of hundreds of genomes renders testable many of these hypotheses, notably that IS abundance correlates positively with the frequency of horizontal gene transfer (HGT), genome size, pathogenicity, nonobligatory ecological associations, and human association. We thus reannotated ISs in 262 prokaryotic genomes and tested these hypotheses showing that when using appropriate controls, there is no empirical basis for IS family specificity, pathogenicity, or human association to influence IS abundance or density. HGT seems necessary for the presence of ISs, but cannot alone explain the absence of ISs in more than 20% of the organisms, some of which showing high rates of HGT. Gene transfer is also not a significant determinant of the abundance of IS elements in genomes, suggesting that IS abundance is controlled at the level of transposition and ensuing natural selection and not at the level of infection. Prokaryotes engaging in obligatory associations have fewer ISs when controlled for genome size, but this may be caused by some being sexually isolated. Surprisingly, genome size is the only significant predictor of IS numbers and density. Alone, it explains over 40% of the variance of IS abundance. Because we find that genome size and IS abundance correlate negatively with minimal doubling times, we conclude that selection for rapid replication cannot account for the few ISs found in small genomes. Instead, we show evidence that IS numbers are controlled by the frequency of highly deleterious insertion targets. Indeed, IS abundance increases quickly with genome size, which is the exact inverse trend found for the density of genes under strong selection such as essential genes. Hence, for ISs, the bigger the genome the better.

Functional domains of the IS1 transposase: analysis in vivo and in vitro

The IS1 bacterial insertion sequence family, considered to be restricted to Enterobacteria, has now been extended to other Eubacteria and to Archaebacteria, reviving interest in its study. To analyse the functional domains of the InsAB' transposase of IS1A, a representative of this family, we used an in vivo system which measures IS1-promoted rescue of a temperature-sensitive pSC101 plasmid by fusion with a pBR322::IS1 derivative. We also describe the partial purification of the IS1 transposase and the development of several in vitro assays for transposase activity. These included a DNA band shift assay, a transposase-mediated cleavage assay and an integration assay. Alignments of IS family members (http://www-is.biotoul.fr) not only confirmed the presence of an N-terminal helix-turn-helix and a C-terminal DDE motif in InsAB', but also revealed a putative N-terminal zinc finger. We have combined the in vitro and in vivo tests to carry out a functional analysis of InsAB' using a series of site-directed InsAB' mutants based on these alignments. The results demonstrate that appropriate mutations in the zinc finger and helix-turn-helix motifs result in loss of binding activity to the ends of IS1 whereas mutations in the DDE domain are affected in subsequent transposition steps but not in end binding.

Presence of a characteristic D-D-E motif in IS1 transposase

Transposases encoded by various transposable DNA elements and retroviral integrases belong to a family of proteins with three conserved acidic amino acids, D, D, and E, constituting the D-D-E motif that represents the active center of the proteins. IS1, one of the smallest transposable elements in bacteria, encodes a transposase which has been thought not to belong to the family of proteins with the D-D-E motif. In this study, we found several IS1 family elements that were widely distributed not only in eubacteria but also in archaebacteria. The alignment of the transposase amino acid sequences from these IS1 family elements showed that out of 14 acidic amino acids present in IS1 transposase, three (D, D, and E) were conserved in corresponding positions in the transposases encoded by all the elements. Comparison of the IS1 transposase with other proteins with the D-D-E motif revealed that the polypeptide segments surrounding each of the three acidic amino acids were similar. Furthermore, the deduced secondary structures of the transposases encoded by IS1 family elements were similar to one another and to those of proteins with the D-D-E motif. These results strongly suggest that IS1 transposase has the D-D-E motif and thus belongs to the family of proteins with the D-D-E motif. In fact, mutant IS1 transposases with an amino acid substitution for each of the three acidic amino acids possibly constituting the D-D-E motif were not able to promote transposition of IS1, supporting this hypothesis. The D-D-E motif identified in IS1 transposase differs from those in the other proteins in that the polypeptide segment between the second D and third E in IS1 transposase is the shortest, 24 amino acids in length. Because of this difference, the presence of the D-D-E motif in IS1 transposase has not been discovered for some time.

The basis of asymmetry in IS2 transposition

In the first step of IS2 transposition, the formation of an IS2 minicircle, the roles of the two IS ends differ. Terminal cleavage initiates exclusively at the right inverted repeat (IRR) - the donor end - whereas IRL is always the target. At the resulting minicircle junction, the two abutted ends are separated by a spacer of 1 or 2 basepairs. In this study, we have identified the determinants of donor and target function. The inability of IRL to act as a donor results largely from two sequence differences between IRL and IRR - an extra basepair between the conserved transposase binding sequences and the end of the element, and a change of the terminal dinucleotide from CA-3' to TA-3'. These two changes also impose a characteristic size on the minicircle junction spacer. The only sequences required for the efficient target function of IRL appear to be contained within the segment from position 11-42. Although IRR can function as a target, its shorter length and additional contacts with transposase (positions 1-7) result in minicircles with longer, and inappropriate, spacers. We propose a model for the synaptic complex in which the terminus of IRL makes different contacts with the transposase for the initial and final strand transfer steps. The sequence differences between IRR and IRL, and the behavioural characteristics of IRL that result from them, have probably been selected because they optimize expression of transposase from the minicircle junction promoter, Pjunc.

IS903 transposase mutants that suppress defective inverted repeats

The inverted repeats (IRs) of the insertion element IS903 are composed of two functional regions. An inner region, consisting of basepairs 6-18, is the transposase binding site. The outer region (positions 1-3) is not contacted during initial transposase binding, but is essential for efficient transposition. We have examined the interaction of the IR with the transposase by isolating transposase suppressors of IR mutations. These suppressors define two patches within the N-terminus of the protein. One class of suppressors, which rescued the majority of outer IR mutants tested, contained mutations in close proximity to an aspartate residue (D121) believed to form part of the catalytic DDE motif, suggesting that their suppressive effect is in the positioning of the catalytic site at the terminus of the transposon. The hypertransposition phenotype of mutant VA119 is also consistent with this hypothesis. The second class was more allele specific and preferentially suppressed a mutation at position 3 of the IR. Finally, we showed that mutations at the termini of the IR elevate the frequency of cointegrate formation by IS903. Other outer IR mutations did not have this effect. These data are consistent with the terminal bases of the transposon playing multiple and distinct roles in transposition.

Insertion sequences

Insertion sequences (ISs) constitute an important component of most bacterial genomes. Over 500 individual ISs have been described in the literature to date, and many more are being discovered in the ongoing prokaryotic and eukaryotic genome-sequencing projects. The last 10 years have also seen some striking advances in our understanding of the transposition process itself. Not least of these has been the development of various in vitro transposition systems for both prokaryotic and eukaryotic elements and, for several of these, a detailed understanding of the transposition process at the chemical level. This review presents a general overview of the organization and function of insertion sequences of eubacterial, archaebacterial, and eukaryotic origins with particular emphasis on bacterial elements and on different aspects of the transposition mechanism. It also attempts to provide a framework for classification of these elements by assigning them to various families or groups. A total of 443 members of the collection have been grouped in 17 families based on combinations of the following criteria: (i) similarities in genetic organization (arrangement of open reading frames); (ii) marked identities or similarities in the enzymes which mediate the transposition reactions, the recombinases/transposases (Tpases); (iii) similar features of their ends (terminal IRs); and (iv) fate of the nucleotide sequence of their target sites (generation of a direct target duplication of determined length). A brief description of the mechanism(s) involved in the mobility of individual ISs in each family and of the structure-function relationships of the individual Tpases is included where available.

Genetic analyses of the interactions of the IS1-encoded proteins with the left end of IS1 and its insertion hotspot

Insertion sequence IS1 specifies the InsA, delta InsA-B'-InsB and InsA-B'-InsB protein species. These three proteins have the identical alpha-helix-turn-alpha-helix motif that is likely to be responsible for DNA binding. In fact, InsA binds to the ends of IS1, and regulates gene expression and transposition of IS1. delta InsA-B'-InsB and/or InsA-B'-InsB has been thought to possess a transposase-like activity. Here, I examined the actions of these proteins in vivo on the promoter (pinsL) in the left end of IS1. InsA repressed pinsL-driven gene expression, both in cis and in trans. delta InsA-B'-InsB inhibited it efficiently only when pinsL was located near the construct where delta InsA-B'-InsB is expressed. Furthermore, it has been shown that the possible -10 sequence of pinsL is required for delta InsA-B'-InsB to act on, but the -35 sequence where InsA binds specifically, is not. InsA-B'-InsB appeared not to work on a nearby pinsL. The cis-action of delta InsA-B'-InsB is consistent with the previous observation that the IS1 transposase acts preferentially in cis. Interestingly, delta InsA-B'-InsB acted on a nearby P3 promoter in the IS1 insertion hotspot, and on another promoter outside the hotspot. delta InsA-B'-InsB may generally interact with the regions in or around promoters owing to their low DNA helix stability. Note that IS1 transposes preferentially into A + T-rich DNA segments, and that DNA is unwound from the -10 region of a promoter in transcription. The cis-preference of delta InsA-B'-InsB would result in an overall reduction of transposition of IS1 and its defective copy in a cell, allowing stable existence of the element in its bacterial host.

Detection of an IS2-encoded 46-kilodalton protein capable of binding terminal repeats of IS2

The genome of the transposable element IS2 contains five open reading frames that are capable of encoding proteins greater than 50 amino acids; however, only one IS2 protein of 14 kDa had been detected. By replacing the major IS2 promoter located in the right terminal repeat of IS2 with the T7 promoter to express IS2 genes, we have detected another IS2 protein of 46 kDa. This 46-kDa protein was designated InsAB'. Analyses of the InsAB' sequence revealed motifs that are characteristic of transposases of other transposable elements. InsAB' has the ability to bind both terminal repeat sequences of IS2. It was shown to bind a 27-bp sequence (5'-GTTAAGTGATAACAGATGTCTGGAAAT-3', positions 1316 to 1290 by our numbering system [16 to 42 by the previous numbering system]) located at the inner end of the right terminal repeat and a 31-bp sequence (5'-TTATTTAAGTGATATTGGTTGTCTGGAGATT-3', positions 46 to 16 [1286 to 1316]), including the last 27 bp of the inner end and the adjacent 4 bp of the left terminal repeat of IS2. This result suggests that InsAB' is a transposase of IS2. Since there is no open reading frame capable of encoding a 46-kDa protein in the entire IS2 genome, this 46-kDa protein is probably produced by a translational frameshifting mechanism.

The organization of the outside end of transposon Tn5

The end sequences of the IS50 insertion sequence are known as the outside end (OE) and inside end. These complex ends are related but nonidentical 19-bp sequences that serve as substrates for the activity of the Tn5 transposase. Besides providing the binding site of the transposase, the end sequences of a transposon contain additional types of information necessary for transposition. These additional properties include but are not limited to host protein interaction sites and sites that program synapsis and cleavage events. In order to delineate the properties of the IS50 ends,the base pairs involved in the transposase binding site have been defined. This has been approached through performing a variety of in vitro analyses: a ++hydroxyl radical missing-nucleoside interference experiment, a dimethyl sulfate interference experiment, and an examination of the relative binding affinities of single-site end substitutions. These approaches have led to the conclusion that the transposase binds to two nonsymmetrical regions of the OE, including positions 6 to 9 and 13 to 19. Proper binding occurs along one face of the helix, over two major and minor grooves, and appears to result in a significant bending of the DNA centered approximately 3 bp from the donor DNA-OE junction.

Genetic analysis of the terminal 8-bp inverted repeats of transposon Tn7

Mutations in the terminal 8-bp (5'-T1G2T3G4G5G6C7G8-3') of the inverted repeats of the bacterial transposon, Tn7, were analysed by measuring Tn7 transposition to the attachment site, attTn7. The mutation, C2, present at either end of Tn7 reduces transposition only threefold, but in the double mutant, with C2 at both ends of Tn7, no transposition is detected. C6 mutations have no effect on transposition frequency. Replacement with 5'-A3C4G5C6G7C8-3' at the right end of Tn7 apparently abolishes transposition; yet in the double mutant, where the inverted repeats are restored by substituting this sequence at both ends of Tn7, transposition is partially rescued. This suggests that the mechanism of Tn7 transposition requires communication between the two ends. Tn7 transposition has always been seen to generate a 5-bp target duplication. This is presumed to result from a staggered cut, plus repair synthesis during transposition. We found that two of our right-end mutants, C2 and C6, sometimes yielded a 6-bp target duplication. This observation implies that cleavage of the target site might also involve interaction with the donor ends which, when mutant, relax the specificity for target-site cleavage.

Mutual stabilisation of bacteriophage Mu repressor and histone-like proteins in a nucleoprotein structure

Integration host factor (IHF) binds in a sequence-specific manner to the bacteriophage Mu early operator. It participates with bound Mu repressor, c, in building stable, large molecular mass nucleoprotein complexes in vitro and enhances repression of early transcription in vivo. We demonstrate that, when the specific IHF binding site with the operator is mutated, the appearance of large molecular mass complexes still depends on IHF and c, but the efficiency of their formation is reduced. Moreover, the IHF-like HU protein, which binds DNA in a non-sequence-specific way, can substitute for IHF and participate in complex formation. Since the complexes require both c and a host factor (IHF or HU), the results imply that these proteins stabilise each other within the nucleoprotein structures. These results suggest that IHF and HU are directed to the repressor-operator complexes, even in the absence of detectable sequence-specific binding. This could be a consequence of their preferential recognition of DNA containing a distortion such as that introduced by repressor binding to the operator. The histone-like proteins could then stabilise the nucleoprotein complexes simply by their capacity to maintain a bend in DNA rather than by specific protein-protein interactions with c. This model is supported by the observation that the unrelated eukaryotic HMG-1 protein, which exhibits a similar marked preference for structurally deformed DNA, is also able to participate in the formation of higher-order complexes with c and the operator DNA.

Is the IS1 transposase, InsAB', the only IS1-encoded protein required for efficient transposition?

The transposase of the bacterial insertion sequence IS1 is normally expressed by inefficient translational frameshifting between an upstream reading frame which itself specifies a transposition inhibitor, InsA, and a second consecutive reading frame located immediately downstream. A fused-frame mutant which carries an additional base pair inserted at the point of frameshifting was constructed. This mutant exhibits high transposition activity and should express the transposase, InsAB', constitutively without frameshifting. Unexpectedly, a second protein species was observed to be expressed from this mutant. We demonstrate here that this protein, InsA*, results from continued frameshifting on the modified frameshift motif. The protein retains the activities of the repressor InsA. Its elimination, by further modification of the frameshift motif, results in a further increase in various transposition activities of IS1. These results support the hypothesis that a single IS1-encoded protein, InsAB', is necessary for transposition.

Isolation of a novel IS3 group insertion element and construction of an integration vector for Lactobacillus spp

An insertion sequence (IS) element from Lactobacillus johnsonii was isolated, characterized, and exploited to construct an IS-based integration vector. L. johnsonii NCK61, a high-frequency conjugal donor of bacteriocin production (Laf+) and immunity (Lafr), was transformed to erythromycin resistance (Emr) with the shuttle vector pSA3. The NCK61 conjugative functions were used to mobilize pSA3 into a Laf- Lafs EMs recipient. DNA from the Emr transconjugants transformed into Escherichia coli MC1061 yielded a resolution plasmid with the same size as that of pSA3 with a 1.5-kb insertion. The gram-positive replication region of the resolution plasmid was removed to generate a pSA3-based suicide vector (pTRK327) bearing the 1.5-kb insert of Lactobacillus origin. Plasmid pTRK327 inserted randomly into the chromosomes of both Lactobacillus gasseri ATCC 33323 and VPI 11759. No homology was detected between plasmid and total host DNAs, suggesting a Rec-independent insertion. The DNA sequence of the 1.5-kb region revealed the characteristics of an IS element (designated IS1223): a length of 1,492 bp; flanking, 25-bp, imperfect inverted repeats; and two overlapping open reading frames (ORFs). Sequence comparisons revealed 71.1% similarity, including 35.7% identity, between the deduced ORFB protein of the E. coli IS element IS150 and the putative ORFB protein encoded by the Lactobacillus IS element. A putative frameshift site was detected between the overlapping ORFs of the Lactobacillus IS element. It is proposed that, similar to IS150, IS1223 produces an active transposase via translational frameshifting between two tandem, overlapping ORFs.

Involvement of Escherichia coli FIS protein in maintenance of bacteriophage mu lysogeny by the repressor: control of early transcription and inhibition of transposition

The Escherichia coli FIS (factor for inversion stimulation) protein has been implicated in assisting bacteriophage Mu repressor, c, in maintaining the lysogenic state under certain conditions. In a fis strain, a temperature-inducible Mucts62 prophage is induced at lower temperatures than in a wild-type host (M. Bétermier, V. Lefrère, C. Koch, R. Alazard, and M. Chandler, Mol. Microbiol. 3:459-468, 1989). Increasing the prophage copy number rendered Mucts62 less sensitive to this effect of the fis mutation, which thus seems to depend critically on the level of repressor activity. The present study also provides evidence that FIS affects the control of Mu gene expression and transposition. As judged by the use of lac transcriptional fusions, repression of early transcription was reduced three- to fourfold in a fis background, and this could be compensated by an increase in cts62 gene copy number. c was also shown to inhibit Mu transposition two- to fourfold less strongly in a fis host. These modulatory effects, however, could not be correlated to sequence-specific binding of FIS to the Mu genome, in particular to the strong site previously identified on the left end. We therefore speculate that a more general function of FIS is responsible for the observed modulation of Mu lysogeny.

Translational frameshifting in the control of transposition in bacteria

The expression of an increasing number of genes of both prokaryotic and eukaryotic origin has been shown to be regulated at the translational level by programmed (sequence-specific) ribosomal frameshifting. Among these are the bacterial insertion sequences IS1 and two members of the widely distributed IS3-family, IS150 and IS911. Frameshifting provides a means of specifying several proteins with different functions using a minimum of genetic information. In this review, we survey present understanding of the way in which frameshifting is integrated into the overall control of transposition activity in these elements.

Use of gel retardation to analyze protein-nucleic acid interactions

Protein-nucleic acid interactions are crucial in the regulation of many fundamental cellular processes. The nature of these interactions is susceptible to analysis by a variety of methods, but the combination of high analytical power and technical simplicity offered by the gel retardation (band shift) technique has made this perhaps the most widely used such method over the last decade. This procedure is based on the observation that the formation of protein-nucleic complexes generally reduces the electrophoretic mobility of the nucleic acid component in the gel matrix. This review attempts to give a simplified account of the physical basis of the behavior of protein-nucleic acid complexes in gels and an overview of many of the applications in which the technique has proved especially useful. The factors which contribute most to the resolution of the complex from the naked nucleic acid are the gel pore size, the relative mass of protein compared with nucleic acid, and changes in nucleic acid conformation (bending) induced by binding. The consequences of induced bending on the mobility of double-strand DNA fragments are similar to those arising from sequence-directed bends, and the latter can be used to help characterize the angle and direction of protein-induced bends. Whether a complex formed in solution is actually detected as a retarded band on a gel depends not only on resolution but also on complex stability within the gel. This is strongly influenced by the composition and, particularly, the ionic strength of the gel buffer. We discuss the applications of the technique to analyzing complex formation and stability, including characterizing cooperative binding, defining binding sites on nucleic acids, analyzing DNA conformation in complexes, assessing binding to supercoiled DNA, defining protein complexes by using cell extracts, and analyzing biological processes such as transcription and splicing.

IS231D, E and F, three new insertion sequences in Bacillus thuringiensis: extension of the IS231 family

IS231 constitutes a family of insertion sequences widespread among Bacillus thuringiensis subspecies. Three new IS231 variants have been isolated from B. thuringiensis subspecies finitimus (IS231 D and E) and israelensis (IS231F). Like the previously described IS231A, B and C, these 1.7 kb elements display single open reading frames encoding 477/478-amino-acid proteins which share between 72% and 88% identity with those of the other members of the family. Sequence comparisons also reveal that all the iso-IS231 terminal inverted repeats are strongly conserved 20 bp sequences. A region susceptible to forming a stable hairpin structure is found just upstream of the open reading frame. Nucleotide substitutions occurring on one strand of the hairpin stems are compensated for by complementary changes at facing positions, giving credence to the hypothesis that this secondary structure plays a role in the regulation of transposition. Examination of IS231 D, E and F flanking sequences reveals that IS231F is bordered by a 12 bp direct repeat. No direct repeats were found flanking IS231D or IS231E.

Programmed translational frameshifting and initiation at an AUU codon in gene expression of bacterial insertion sequence IS911

The proteins expressed by insertion sequence IS911, a member of the widespread IS3 family of elements, have been analyzed. The results indicate that three major species are produced from two consecutive reading frames. A protein of Mr 11,500, ORFA, is synthesized from an upstream reading frame. A larger protein, ORFAB, uses the same initiation codon and is produced by a -1 programmed translational frameshift between orfA and a downstream frame, orfB, whose amino acid sequence shows significant homology with retroviral integrase proteins. The orfB frame is also expressed independently in two alternative forms: the first uses a rare AUU initiation codon in the orfB phase whereas the second appears to initiate in the orfA phase and is produced by a -1 frameshift mechanism similar to that used in ORFAB expression. A specific IS911 integration reaction using a minimal active junction composed of 51 base-pairs of the right inverted repeat and a flanking phase lambda sequence resembling a second end in inverted orientation has been developed to analyze the functions of these proteins by transcomplementation in vivo. The orfA and orfB frames are shown to be essential and production of ORFAB is shown to stimulate integration in this system, suggesting that this fusion protein is the IS911 transposase.