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Gene targeting using randomly inserted group II introns (targetrons) recovered from an Escherichia coli gene disruption library

The Lactococcus lactis Ll.LtrB group II intron retrohomes by reverse-splicing into one strand of a double-stranded DNA target site, while the intron-encoded protein cleaves the opposite strand and uses it to prime reverse transcription of the inserted intron RNA. The protein and intron RNA function in a ribonucleoprotein particle, with much of the DNA target sequence recognized by base-pairing of the intron RNA. Consequently, group II introns can be reprogrammed to insert into specific or random DNA sites by substituting specific or random nucleotide residues in the intron RNA. Here, we show that an Escherichia coli gene disruption library obtained using such randomly inserting Ll.LtrB introns contains most viable E.coli gene disruptions. Further, each inserted intron is targeted to a specific site by its unique base-pairing regions, and in most cases, could be recovered by PCR and used unmodified to obtain the desired single disruptant. Additionally, we identified a subset of introns that insert at sites lacking T+5, a nucleotide residue critical for second-strand cleavage. All such introns tested individually gave the desired specific disruption, some by switching to an alternate retrohoming mechanism targeting single-stranded DNA and using a nascent lagging DNA strand to prime reverse transcription.


INTRODUCTION
Mobile group II introns use a remarkable mobility mechanism, termed retrohoming, in which the excised intron RNA uses its ribozyme activity to insert directly into a DNA target site by reverse-splicing and is then reverse-transcribed by the intronencoded protein (IEP) (1,2). Retrohoming is mediated by a ribonucleoprotein (RNP) complex that contains the IEP and the excised intron RNA, with both used for DNA target site recognition (3,4). Because the IEP recognizes only a small number of fixed positions and most of the target specificity comes from base-pairing of the intron RNA to the DNA target sequence, it is possible to reprogram group II introns to insert into desired sites simply by modifying the intron RNA (5,6). This feature combined with their very high insertion frequencies and specificity have made it possible to develop mobile group II introns into gene targeting vectors, dubbed 'targetrons' [reviewed in (7)]. A targetron derived from the Lactococcus lactis Ll.LtrB intron has been used for chromosomal gene disruption and site-specific DNA insertion in a variety of Gram-negative and Gram-positive bacteria (8)(9)(10)(11)(12).
The RNPs that mediate retrohoming are formed when the IEP binds to the intron in unspliced precursor RNA and promotes its splicing by stabilizing the catalytically active RNA structure; it then remains bound to the excised intron lariat RNA in a stable RNP complex (13)(14)(15). To initiate mobility, group II intron RNPs bind DNA and recognize target sites (16). The DNA target site for the Lactococcus lactis Ll.LtrB intron used in the present work is shown in Figure 1A (4,10). The initial recognition event is thought to involve major groove interactions between the IEP and a small number of specific bases in the distal 5 0 -exon region, including T-23, G-21 and A-20. These base interactions, bolstered by neighboring phosphate-backbone interactions and possibly minor groove interactions between positions À18 and À14, lead to local DNA unwinding, enabling the intron RNA to base-pair to the adjacent 14-16 nt DNA sequence for reverse-splicing into the intron-insertion site. The intron RNA sequences involved in base-pairing are located in two RNA stem-loops and are denoted exon-binding sites 1 and 2 (EBS1 and EBS2) and d, while the complementary DNA target-site sequences are denoted intron-binding sites 1 and 2 (IBS1 and IBS2) in the 5 0 exon and d 0 in the 3 0 exon. Second-strand cleavage to generate the primer for reverse transcription requires additional interactions between the IEP and the 3 0 exon, the most critical being recognition of T+5 (4,6). In the absence of second-strand cleavage, the Ll.LtrB intron has been shown to retrohome at low frequency by using nascent leading or lagging strands at DNA replication forks to prime reverse-transcription (17), and such mechanisms are also used for retrohoming by group II introns that encode proteins lacking the DNA endonuclease (En) domain (referred to as En À introns) (18). In all cases, the IEP is thought to synthesize a full-length intron cDNA, which is then integrated into the recipient DNA by a DNA repair mechanism independent of homologous recombination (19)(20)(21).
For use in gene targeting, the Ll.LtrB intron (targetron) is expressed from a donor plasmid, such as pACD3 ( Figure 1B) (5,9). This plasmid uses an inducible T7lac promoter to express a DORF-derivative of the intron and short flanking exons, with the LtrA ORF cloned just downstream of the 3 0 exon. The LtrA protein expressed from this position still binds to the intron to promote RNA splicing and mobility, but when the DORF intron integrates at a new location, it is unable to splice in the absence of the IEP, yielding a gene disruption. Currently, the targetron is programmed to insert into different sites with the help of a computer algorithm, which scans the target sequence for the best matches to the positions recognized by the IEP and then designs primers from modifying the intron's EBS1, EBS2 and d sequences to insert into those sites (10). The positions recognized by the IEP are sufficiently few and flexible that the algorithm readily identifies multiple rank-ordered target sites in any gene. The IBS1 and IBS2 The DNA target site for the Ll.LtrB intron is recognized by an RNP containing the IEP (LtrA protein) and excised intron lariat RNA, with both the protein and base-pairing of the intron RNA used to recognize the target sequence. Key bases recognized by the IEP (gray shading) include TÀ23, GÀ21 and AÀ20 in the 5 0 exon and T+5 in the 3 0 exon; T+5 is required only for second-strand cleavage (5,6,10,12). The intron RNA's EBS2, EBS1 and d sequences base-pair with IBS2, IBS1 and d 0 sequences located between DNA target site positions À12 and +3. IS and CS (arrowheads) indicate the intron-insertion site and second-strand cleavage site, respectively. (B) Intron-donor plasmid pACD3 contains a 0.9 kb Ll.LtrB-DORF intron and flanking exons cloned downstream of a T7lac promoter, with the LtrA protein expressed from a position just downstream of the 3 0 exon (9). The intron is retargeted by modifying its EBS2, EBS1 and d sequences to base-pair to the IBS2, IBS1 and d 0 sequences in the DNA target site. The IBS1 and IBS2 sequences in the donor plasmid are also modified to base-pair to the intron's retargeted EBS1 and EBS2 sequences for efficient RNA splicing. The required modifications are introduced into the donor plasmid via a two-step PCR using primers P 1 to P 4 , as diagrammed in the figure (5,9). (C) The intron-donor plasmid pACD3-Tp-RAM contains a retrotransposition-activated Tp R gene (Tp R -RAM) (12). The Tp R gene is inserted in group II intron DIV in the reverse orientation, but interrupted by an efficiently self-splicing group I intron (the phage T4 td intron) in the forward orientation. During retrotransposition via an RNA intermediate, the group I intron is spliced reconstituting the marker, which can then be selected after integration into a DNA target site. sequences in the 5 0 exon of the donor plasmid are also modified to be complementary to the retargeted EBS1 and EBS2 sequences for efficient RNA splicing. The necessary modifications are introduced into the donor plasmid by a two-step PCR, using three unique primers (P 1 , P 2 and P 4 ) and one 'fixed' primer (P 3 ; Figure 1B).
Targetrons that insert at the desired site can be detected by a resulting phenotypic change, by colony PCR or by using a genetic marker inserted in intron domain IV, a non-essential region that extends outside of the intron's catalytic core. Particularly useful for gene targeting is a Retrotransposition Activated Marker (RAM) (12); also referred to as a Retrotransposition-Indicator Gene or RIG (22). The RAM is a selectable gene, such as trimethoprim-resistance (Tp R ), inserted in the group II intron in the reverse orientation, but interrupted by an efficiently self-splicing group I intron in the forward orientation ( Figure 1C). During retrotransposition via an RNA intermediate, the group I intron is spliced allowing direct selection of the marker after DNA integration (12).
In addition to targeted disruption, the incorporation of a RAM makes it possible to use a targetron with randomized target-site recognition (EBS1, EBS2 and d) sequences to obtain a gene disruption library in which group II introns integrate at sites distributed throughout a genome, analogous to global transposon mutagenesis (12). Here, we show that an E.coli gene disruption library obtained using this approach contains a high proportion of all viable disruptions. Further, unlike a conventional transposon, each of the inserted targetrons in the library is potentially targeted to a specific location by virtue of its unique EBS1, EBS2 and d sequences, and in most cases, could be recovered ('fished') by PCR and inserted into a donor plasmid to give the desired single disruptant. Additionally, we identified a subset of introns that could be targeted to sites that lack T+5, including two that retrohome at relatively high frequency, apparently by targeting singlestranded DNA and using a nascent lagging DNA strand to prime reverse-transcription.

MATERIALS AND METHODS
Bacterial strains and growth condition E.coli HMS174 (DE3) (F À recA hsdR Rif R ) (Novagen, Madison, WI), which contains an isopropyl-b-D-thiogalactoside (IPTG)-inducible phage T7 RNA polymerase, was used for generating the chromosomal gene disruption library and for individual gene disruption experiments. DH5a was used for cloning. LB medium was used for most experiments, with antibiotics added as required at the following concentrations: ampicillin, 100 mg/ml; chloramphenicol, 25 mg/ml; and tetracycline, 25 mg/ml. Trimethoprim selection was done in Mueller-Hinton or M9 medium supplemented with 10 mg/ml trimethoprim and 1 mg/ml thymine, which was found necessary for selection of the chromosomally integrated Tp R marker (12).

Recombinant plasmids
pACD3 contains a 0.9-kb Ll.LtrB-DORF intron and flanking exons cloned behind a T7lac promoter, with the IEP expressed from a position just downstream of the 3 0 exon (9). pACD3-Tp-RAM (previously denoted pACD3-RAM) is a derivative of pACD3 in which a retrotransposition-activated Tp R marker was inserted in intron domain IV (12). pACD3E and pACD3E-Tp-RAM are derivatives of the above plasmids in which 5 0 exon-position À18 to intron position +289 (positions numbered from the 5 0 -splice site) were deleted and replaced with an EcoRV site.
The E.coli gene disruption library was generated using a pACD3-Tp-RAM donor intron modified to contain randomized nucleotide residues at EBS2 positions À12 to À8, EBS1 positions À6 to À1 and d positions +1 to +3 (12). The corresponding IBS1 and IBS2 positions in the 5 0 exon of the donor plasmid were also randomized to obtain nucleotide combinations complementary to the randomized EBS1 and EBS2 sequences for RNA splicing.
Mobility assays to determine the effect of DNA target site orientation on retrohoming frequency were done with donor plasmid pACD2X, which contains a 940 nt Ll.LtrB-DORF with a phage T7 promoter inserted near its 3 0 end (23). The recipient plasmids were derivatives of pBRR3A-ltrB and pBRR3B-ltrB, which contain the Ll.LtrB target site upstream of a promoterless tet R gene in opposite orientations relative to the direction of plasmid DNA replication (17). DNA target sites to be tested (positions À30 to +15 from the intron-insertion site) were synthesized as doublestranded DNA oligonucleotides with appended AatII and EcoRI sites and swapped for the wild-type Ll.LtrB target site between the corresponding sites of pBRR3A-ltrB and pBRR3B-ltrB.

E.coli chromosomal gene disruption library
The E.coli gene disruption library was generated by using the pACD3-Tp-RAM-based donor intron with randomized EBS2, EBS1 and d sequences, as described previously (12). Briefly, 5 mg of the donor plasmid was electroporated into E.coli HMS174(DE3), and transformants were grown overnight at 37 C in LB medium containing chloramphenicol: 10 ml of the overnight culture was then inoculated in 250 ml of fresh LB medium, and cells were grown to log phase for 3 h at 37 C. To induce intron expression, a 15 ml sample of these cells was added to 250 ml of LB containing 500 mM IPTG and incubated at 30 C for 18 h. A 10 ml sample of these cells was then washed and grown to saturation in 250 ml of Mueller-Hinton medium containing trimethoprim (10 mg/ml) plus thymine (1 mg/ml). Chromosomal DNA was isolated from 30 ml of the culture using a Genomic-tip and Genomic DNA Buffer set (Qiagen, Valencia, CA).

Targetron fishing
Targetrons were 'fished' from the E.coli gene disruption library DNA by PCR. The PCR for targetrons inserted into the sense and antisense strands used target gene-specific primers (P s and P a ), which base-pair to 5 0 -and 3 0 -flanking sequences, respectively, together with a fixed intron primer (P i ) 5 0 -TCAGATTCTCGGCATCGCTTTCGTTTC, which base-pairs to a sequence downstream of EBS1 ( Figure 3). The PCRs were carried out in 40 ml of reaction medium containing 4 U of PlatinumTaq (Invitrogen, Carlsbad, CA), 1 mg of E.coli chromosomal DNA and 1 mM of each primer for 30 cycles, with the annealing temperatures and extension times optimized for the size of each target gene and the T m for each primer. PCR products were gel-purified and sequenced. For cloning into the donor plasmid, the PCR products were digested with BsrGI plus a second restriction enzyme that generates a blunt end within the 5 0 -exon of the target gene, and then cloned into the vector backbones of pACD3 or pACD3-Tp-RAM by ligating to EcoRV + BsrGI-digested pACD3E or pACD3E-Tp-RAM, respectively. Target gene-specific primers used in this study are summarized in Table 1.

Use of targetrons for chromosomal gene disruption
For chromosomal gene disruptions, pACD3-based plasmids containing fished targetrons were transformed into E.coli HMS174(DE3) and grown overnight in LB medium containing chloramphenicol: 50 ml of the overnight culture was inoculated into 5 ml of fresh LB medium containing chloramphenicol and grown at 37 C until OD 598 was 0.2, then induced with 500 mM IPTG for 18 h at 30 C (12). The cells were washed once with fresh LB by centrifugation at 3750 g for 5 min at 4 C, resuspended in 5 ml of fresh LB, and then plated on LB and incubated overnight at 37 C. Disruptants were identified by colony PCR using primers flanking the gene and confirmed by DNA sequencing. Gene disruptions with pACD3-Tp-RAM-based donor plasmids were done similarly, except that after induction, the cells were grown in 5 ml of M9 medium without antibiotics for 2 h at 30 C, and then plated on M9 medium containing trimethoprim plus thymine, and incubated for 2-3 days at 30 C.

PCR for direct insertion of fished targetrons into donor plasmids without cloning
Targetrons were fished from the chromosomal gene disruption library as above, by using the intron primer P i , together with a gene-specific primer, which is complementary to 5 0 -or 3 0flanking sequences, but with an additional 27-nt vector sequence (5 0 -CCATTCCCCTCTAGAAAAAAGCTTCGT) appended to its 5 0 end. The vector sequence is located upstream of the EcoRV site of pACD3-Tp-RAM-PCR. The PCR products were purified by using a PCR cleanup kit (Qiagen) and resuspended in 50 ml of distilled water. The first-round PCR products (2 ml) were then combined with EcoRV-digested pACD3-Tp-RAM-PCR by an additional PCR, yielding a non-covalently closed donor plasmid, which was transformed directly into E.coli for gene disruption.

Southern hybridization
Disruptants were transformed with pACYC177 and grown in LB medium containing ampicillin to kickout the introndonor plasmid (confirmed by acquisition of Amp R and loss of Cam R ). The disruptants were then grown up in liquid culture, and chromosomal DNA was isolated by using a Qiagen Genomic-tip and Genomic DNA Buffer set. Southern hybridization was done as described in (10), using a 32 P-labeled probe for the retrotransposed intron generated by PCR of pACD3 with primers 5 0 -TCTTGCAAGGGTACGGAGTA and 5 0 -GTAGGGAGGTACCGCCTTGTTC. The probe was labeled using a High Prime DNA Labeling kit (Roche Diagnostics, Indianopolis, IN).
Plasmid-based mobility assays to determine the effect of target-site orientation on retrohoming frequency Intron-donor plasmid pACD2X and recipient plasmids pBRR3A-ltrB, pBRR3B-ltrB or their derivatives containing different target sites were cotransformed into E.coli HMS174(DE3) (17). After induction with 100 mM IPTG for 1 h at 37 C, mobility events were detected by plating different dilutions of cells onto LB medium containing ampicillin or ampicillin plus tetracycline. The plates were incubated at 37 C for 24 h and mobility frequencies were calculated from the ratio of (Tet R + Amp R )/Amp R colonies.

RESULTS AND DISCUSSION
Generation of an E.coli gene disruption library using randomly inserting targetrons To explore the efficacy of targetron fishing, we generated an E.coli gene disruption library by using a previously constructed targetron in which the EBS2, EBS1 and d sequences had been randomized (12). The targetron is a 0.9-kb Ll.LtrB-DORF intron, containing a Tp R -RAM gene, and is cloned behind a T7lac promoter in the donor plasmid  Figure 1B). After transformation into E.coli HMS174(DE3), which contains an IPTG-inducible phage T7 RNA polymerase, cells were induced with IPTG and grown to saturation in Mueller-Hinton medium containing trimethoprim to select those containing integrated targetrons. This outgrowth step selects against targetrons that have inserted into essential genes or whose disruption results in a decreased growth rate, but in principle such targetrons could be obtained by isolating cells sooner after induction.
Genomic DNA was isolated from the trimethoprim-enriched cell culture and used as a template for PCR to fish targetrons that inserted into specific genes.

Targetron fishing
We selected an initial set of 20 genes, including 11 genes more or less evenly distributed around the E.coli chromosome (araD, corA, lacZ, lhr, mgtA, narQ, recD, rfc, trpE, uvrB and yhbY), two DNA helicase genes (ruvA and ruvB), the 23S rRNA methyltransferase gene rrmJ and six genes encoding potential RNA-binding proteins of unknown function (b2654, b2856, rraA, ybcJ, ycdV and ygjH). In some cases, the genes were selected because of their relevance to other projects in the laboratory. Fifteen of these genes were known to be non-essential, while the remaining five (b2654, b2856, rraA, ybcJ and yhbY) had not been characterized as essential or non-essential (24) (http://shigen.lab.nig.ac.jp/ecoli/pec/index. jsp). The location of the genes on the E.coli chromosome is shown in Figure 2.
Targetrons were fished from the E.coli gene disruption library by PCR. For each gene, we carried out PCRs using a 'fixed' primer (P i ) complementary to an intron sequence just downstream of EBS1, together with a gene-specific primer in the 5 0 -or 3 0 -flanking region to amplify targetrons that had inserted into the sense or antisense strands, respectively (P s and P a , respectively; Figure 3). Both PCRs yield products that span the 5 0 -integration junction and include the targetron's EBS2, EBS1 and d sequences, as well as the target gene's complementary IBS2 and IBS1 sequences. If more than one targetron has inserted into the gene, the PCR yields multiple products of different sizes. The PCR products were sequenced and cloned into the vector backbones of the donor plasmids pACD3 or pACD3-Tp-RAM (see Materials and Methods).
We successfully fished targetrons that inserted into 17 of the 20 genes tested (8 both strands, 4 sense strand only and 5 antisense strand only; in some cases, only one strand was attempted; see Table 1). Target sequences and base-pairing  . Scheme for targetron fishing. Chromosomal DNA from an E.coli gene disruption library containing randomly inserted Ll.LtrB introns is used as a template for PCR with a 'fixed' primer within the intron (P i ) and target gene-specific primers upstream (sense-strand insertions) or downstream (antisense-strand insertions) of the target gene (P s and P a , respectively). The resulting PCR products span the targetron integration junction and contain the 5 0 -exon sequences IBS1 and IBS2 in the target gene and the target-site recognition sequences EBS2, EBS1 and d in the intron. The PCR product is then ligated or PCRed (see Figure 8) into a donor plasmid and transformed into an E.coli strain to obtain the desired single disruptant. Tp R -RAM is the trimethoprim-resistance retrotransposition-activated marker gene inserted in the Ll.LtrB-DORF intron (see Figure 1C).
interactions for the fished targetrons are summarized in Figures 4 and 5. Multiple targetrons were isolated for araD, lacZ, mgtA, recD, ruvB and ygjH, while only one targetron was isolated for each of other genes. Two targetrons that inserted into 3 0 -untranslated regions were also isolated, one 40 nt downstream of ybcJ (YbcJ+40s) and the other 5 nt downstream of ygjH (YgjH+5a) (Figure 4). In a number of cases, two or more targetrons that differ slightly in their EBS1, EBS2 or d sequences were found inserted at the same site (four in AraD260s, three in MgtA1273s, two each in AraD538a, CorA389s, MgtA1300s, NarQ165s, NarQ1387a, RuvB186a and YhbY84a; different targetrons that insert at the same site are distinguished by subscripts). The frequency of targetron insertion at different sites could be influenced by many factors, including the fitness of the target sequence, the activity of the target gene, partial or complete occlusion of sites by DNA-binding proteins or the frequency of suitable targetrons in the donor-plasmid library. The distance of the target site from the DNA replication origin could also be a factor, although highly efficient targetrons can be obtained for sites distributed throughout the E.coli genome (12).
The three genes for which we were unable to fish targetrons were b2654 and b2856, both of which are very small (333 and 165 bp, respectively) and of unknown essentiality, and rfc (aka wbbH, 1167 bp), which encodes O antigen polymerase (25). Neither the rfc gene nor other genes in the same operon are essential in E.coli K12 MG1655 (24), and we were able to disrupt rfc at low frequency in HMS174(DE3) by using a computationally designed targetron (Rfc706a) with a Tp R -RAM gene (data not shown). Thus, the failure to recover Targetrons are named by nucleotide position 5 0 to their insertion site in the target gene's coding sequence, followed by 's' or 'a' indicating sense or antisense strands, respectively. Nucleotide residues that match the wild-type Ll.LtrB target site are highlighted in black with white letters; arrowheads indicate the targetron-insertion site. Asterisks indicate targetrons that were modified to form Watson-Crick base pairs with the target sequence at previously mispaired positions and to have a favorable A residue at the d+3 position [see (10)]. Different targetrons that inserted at the same site are distinguished by subscripts (see Figure 5 for complete list). LacZ294s (Alt) shows an alternate RNA-DNA basepairing for this targetron in which TÀ4 in the DNA target site is flipped out. The ability of the fished targetron cloned in pACD3 or pACD3-Tp-RAM to give the desired single disruption is indicated under the column 'Targeted Disruption'. Numbers indicate the insertion frequency based on PCR screening of 24-108 colonies for targetrons cloned in pACD3, and + or À indicate successful or unsuccessful disruption, respectively, for targetrons cloned in pACD3-Tp-RAM; nt, not tested.
Rfc targetrons from the library could reflect either that the disruptants were underrepresented initially or that they grow slowly and were lost in the outgrowth step. Figures 6 and 7 show compilations of nucleotide frequencies and base-pairing interactions at the 36 insertion sites of targetrons fished from the library. These data are generally in good agreement with those obtained previously for randomly integrated targetrons isolated from an E.coli gene disruption library by inverse PCR (12). Most of the isolated targetrons have good matches for the critical positions recognized by the IEP, including G-21 (64%) and T+5 (78%), as well as good but not perfect EBS/IBS and d/d 0 pairings (see also Figures 4 and 5). The compilations show that the requirement for base-pairing is most stringent for positions À12 to +1 (81-100% base-pairing), and less stringent at position +2 (72%), while position +3 shows a previously noted strong preference for an A-residue in the intron RNA (73%) regardless of its potential base-pairing partner in the DNA target site (10,12). The apparent preference for the wild-type C-28 (50%) was not seen previously (12) and likely reflects a statistical anomaly in the smaller data set in the present work.

Characteristics of fished targetrons and their insertion sites
Notably, several target sites deviate substantially from the canonical sequence recognized by the IEP, including some that were sites of multiple insertions by different targetrons  ( Figure 5). The latter include the target sites for AraD260s, AraD538a and MgtA1273s, all of which lack T-23, G-21, A-20 and G-17, and those for NarQ165s and NarQ1387a, both of which lack T+5, a critical nucleotide residue for second-strand cleavage. LacZ294s also appears to have a highly deviant target site with an unfavorable G at position À23 and C at position À21, and six mispairings between positions À12 and +1 (Figure 4). However, we noticed that target site could conform to that for the wild-type Ll.LtrB intron if a single T residue at position À4 was flipped-out to change the register by one nucleotide residue [see alternate pairing for LacZ294s (Alt) in Figure 4], and we confirmed that a target site in which T-4 was deleted supported efficient retrohoming in a plasmid assay (data not shown). We show below that all of the targetrons that recognize deviant target sites still gave the expected disruption when tested individually.

Use of fished targetrons to obtain single disruptants
Thirty-one of the fished targetrons were recloned into the pACD3 and/or pACD3-Tp-RAM donor plasmid and transformed into E.coli HMS174(DE3) to see if they would give the desired single disruptant. We note that the target gene's 5 0flanking sequences, which are cloned with the targetron into the donor plasmid (see Figure 3), automatically contain IBS1 and IBS2 sequences complementary to the targetron's unique EBS1 and EBS2 sequences, since these base-pairing interactions were necessary for targetron integration by reverse-splicing (Figures 4, 5 and 7). In the donor construct, base-pairing between the targetron's EBS sequences and the complementary IBS sequences in the 5 0 exon is necessary for RNA splicing to generate active RNPs (26).
Of the 31 targetrons tested, 27 gave the desired single disruptant without further modification (results summarized under 'Targeted Disruption in Figures 4 and 5). In each case, insertion of the targetron at the correct site was confirmed by colony PCR and DNA sequencing (data not shown). The four targetrons that did not give the desired single disruptants, AraD288a, AraD538a 1 , MgtA2516a and RecD1620a, each contains mispairings in the EBS/IBS interactions. When retested after modifying their EBS sequences to form Watson-Crick base pairs at the mispaired positions and to insert a preferred A residue at the d+3 position (targetrons indicated by asterisks in Figure 4), three gave the desired single disruptant. The RecD1620a targetron, which was confirmed to have the correct sequence, did not give the desired single disruption. It appears to have a favorable target site for IEP recognition, but its EBS2 sequence, 5 0 -GGCGC, could mispair with a neighboring sequence just upstream in the intron RNA to disrupt the Id1 stem-loop, which displays EBS2. Such mispairings have been found to result in low targeting efficiencies (10).
In general, more efficient targetrons had better matches to the wild-type target site and higher numbers of EBS/IBS and d/d 0 base pairs than did the less efficient targetrons. However, even targetrons with deviant target sites or significant numbers of mispairings gave specific disruptions. The LacZ294s targetron, mentioned above, gave disruptions at a frequency of 1% by blue-white screening. YhbY84a 2 and MgtA1273s 1 , which have two or three mispairs at key positions, respectively, had targeting efficiencies sufficient to detect using the Tp R -RAM, as did all four YgjH179a targetrons, despite mispairing at different positions in three of the targetrons (Figures 4 and 5). Finally, seven targetrons that insert at sites lacking T+5 were also tested, and all gave the desired specific disruptions (Lhr400s; MgtA606s, MgtA2466a, NarQ165s 1 , NarQ1387a 1 , RecD1682a, and YhbY107s; Figure 4).

A PCR method for inserting fished targetrons into donor plasmids without cloning
For high-throughput approaches, it is desirable to generate donor plasmids containing the fished targetrons without cloning. For this purpose, we developed the procedure diagrammed in Figure 8 in which a 27 nt vector sequence (v) was added to the 5 0 end of the target gene-specific primers used for targetron fishing. The resulting PCR product was then used in a second PCR with EcoRV-digested donor plasmid pACD3E-Tp-RAM-PCR (12), yielding a non-covalently closed circular PCR product, which can be transformed directly into E.coli. This method avoids the use of a gene-specific restriction site flanking the target gene for blunt-end cloning into the vector. A test of this method in which the AraD260s 1 targetron was amplified directly into the pACD3E-Tp-RAM plasmid backbone gave the correct disruption at roughly the same frequency as the conventional cloning method (data not shown).

Specificity of targetron insertion
Because some of the 'fished' targetrons have suboptimal protein recognition and base-pairing interactions, there was a danger that they would be less specific than designed targetrons. To test the specificity of targetron insertion, we carried out Southern hybridizations for a randomly chosen disruptant obtained with each of the fished targetrons that was active in  integration junction of the targetron was generated by PCR using primers P i and P s or P a , the latter two with an additional 5 0 sequence complementary to the vector (v). The PCR product was then used as a primer for a second PCR with the 6.7 kb EcoRV-digested pACD3-Tp-RAM-PCR plasmid as the template. The final circular PCR product contains gaps (arrowheads) at different positions, depending on the strand of the 361 bp linear DNA from which priming occurred. The PCR product can be transformed directly into E.coli or after ligation to seal nicks. giving disruptants. In 22 cases, the Southern hybridizations showed a single prominent band of the size expected for sitespecific insertion of the targetron into the target gene, while in the remaining eight cases (AraD288a*, AraD538a 1 *, Lhr4038a, MgtA606s, MgtA1300s 1 , MgtA2516a*, RuvA51s and YhbY84a 2 ), the initial disruptant showed multiple bands. In these eight cases, we isolated 5-10 additional transformants using a shorter induction period (2 h instead of 18 h; see Materials and Methods), and in each case, we obtained at least one transformant that gave a single integration at the desired site. Figure 9 shows Southern blots for the complete set of single disruptions for each targetron. We note that all seven of the tested targetrons that insert at sites lacking T+5 gave the desired single disruptant, with only one (MgtA606s) being in the group that initially gave multiple bands.

Analysis of retrohoming into target sites that lack T+5
Those targetrons that insert at sites lacking T+5 could do so either by an En-dependent pathway with inefficient secondstrand cleavage or by En-independent pathways using nascent leading or lagging DNA strands to prime reverse-transcription (17,18,22,27). To help distinguish these possibilities, we tested the ability of each targetron to retrohome into its target site cloned in recipient plasmids in opposite orientations relative to the direction of DNA replication. The assay used is diagrammed in Figure 10A. In this assay, the Amp R recipient plasmids pBRR3A and pBRR3B contain the target site cloned immediately upstream of a promoterless tet R in orientations denoted LAG or LEAD, according to whether a nascent lagging or leading DNA strand could be used to prime reversetranscription of the inserted intron RNA (17). The donor intron contains a phage T7 promoter inserted near its 3 0 end, so that its insertion into the target site activates the expression of the tet R gene. Mobility frequencies are then determined from the ratio of (Tet R + Amp R )/Amp R colonies.
In previous work using the above assay, we found that retrohoming of the Ll.LtrB intron under conditions where second-strand cleavage is inhibited by mutations in either the IEP or DNA target site shows a pronounced bias for the LEAD orientation (17). This bias is thought to reflect that when the intron reverse-splices into double-stranded DNA prior to passage of a replication fork, it is positioned to directly use a nascent leading-strand primer, while use of a laggingstrand primer requires the potentially disruptive passage of the replication fork through the inserted intron. In contrast, some En À group II introns are thought to insert preferentially into single-stranded DNA at replication forks, enabling the efficient use of lagging-strand primers, and such introns show the opposite orientation bias in this type of assay (18).
The results of the assays for the wild-type Ll.LtrB intron and those targetrons that insert into sites lacking T+5 are summarized in Figure 10B. As expected, the wild-type Ll.LtrB intron, which uses an En-dependent retrohoming pathway, showed no significant orientation bias. Five of the seven target sites lacking T+5 are located on the leading template strand of the E.coli chromosome. In the plasmid assay, all of these sites were used at either low or undetectable frequency. Three of these sites (Lhr400s, MgtA2466a and RecD1682a) showed a clear bias for insertion into the leading template strand, the pattern expected for the reverse-splicing into double-stranded DNA and use of a nascent leading DNA strand to prime reverse-transcription, while the other two sites (NarQ165s and YhbY107s) showed no significant strand bias, the pattern expected for an En-dependent pathway with inefficient second-strand cleavage.
The remaining two target sites lacking T+5 are located on the lagging template strand of the E.coli chromosome. In the plasmid assay, one of these sites (MgtA606s) showed a very pronounced bias for the lagging template strand, with a relatively high insertion frequency (7.4%) in that orientation, and no detectable insertion in the opposite orientation (<0.0001%). This bias is expected for reverse-splicing into single-stranded DNA and use a nascent lagging strand to prime reversetranscription. The inability of MgtA606s to reverse-splice into double-stranded DNA may reflect suboptimal IEP recognition in the 5 0 exon and/or that it has relatively weak base pairs at EBS2/IBS2 positions À12 and À11 (U-A and A-T), which are thought to be critical for nucleating local DNA unwinding (4,10).
The other site found on the lagging template strand of the E.coli chromosome, NarQ1387a, showed a substantial bias for the LAG orientation (9.6%), but also supports relatively efficient retrohoming in the LEAD orientation (1.2%). This pattern suggests preferential targeting of single-stranded DNA, but substantial reverse-splicing into double-stranded DNA. The ability of the NarQ1387a 1 targetron to efficiently reverse-splice into double-stranded DNA may reflect that its EBS2/IBS2 pairing consists largely of stable GC or CG base pairs (Figure 4), thereby favoring displacement of the opposite strand for reverse-splicing into double-stranded DNA.

SUMMARY AND PROSPECTS
The targetron fishing approach selects targetrons that have successfully inserted into desired target genes and uses them as reagents to obtain specific single disruptions. Despite the previously observed preference for targetron insertion near the chromosome replication origin under similar conditions (12), the library obtained here is sufficiently complex to contain most viable E.coli gene disruptants. The fished targetrons can in principle be used to obtain disruptions in any bacterial strain, so long as the target sequence does not deviate at significant positions. The remarkable finding is that a high proportion of the fished targetrons gave the desired single disruptant without further modification, even if there were mispairings in the EBS/IBS and d-d 0 interactions. By selecting only targetrons that have inserted successfully, the fishing approach avoids factors that could negatively impact introninsertion frequency and have not yet been incorporated into computer algorithms for target site selection and intron design. These include the effect of higher-order DNA structure and thermal stability profiles of the target sites, protein-binding at DNA target sites, and the deleterious effects of different combinations of nucleotide residues on intron RNA structure. Targetron fishing also has the advantage of requiring only two target gene-specific primers and a simple single-step PCR, compared to a more complex PCR for retargeting computationally designed group II introns (10).
The finding that some targetrons retain specificity even in the absence of perfect EBS/IBS and d-d 0 interactions implies a significant degree of redundancy in the RNA and protein interactions used for target site selection. Additionally, we were surprised by the number of the fished targetrons that gave specific disruptions despite substantial deviations from current target site-selection rules (10). One of these, LacZ294s, appears to recognize its target site by flipping out one target site nucleotide residue in the base-paired region, and seven targetrons insert specifically at sites lacking T+5, which is critical for second-strand cleavage. Two of these targetrons appear to insert at relatively high efficiency by targeting single-stranded DNA and using a nascent lagging strand to prime reverse-transcription, a mechanism used by En À group II introns (18,27). Thus, in addition to providing a rapid method for obtaining validated targetrons, our results show that Ll.LtrB can be targeted to a much greater range of sites The Cap R donor plasmid pACD2X expresses a 940 nt Ll.LtrB-DORF intron with a phage T7 promoter inserted near its 3 0 end. The Amp R recipient plasmids pBRR3A and pBRR3B contain a target site/tet R cassette cloned in opposite orientations denoted LAG or LEAD, depending on whether a nascent lagging or leading DNA strand could be used to prime reverse-transcription of the reversespliced intron RNA. Insertion of the intron into the target site activates the expression of the tet R gene, and mobility frequencies are measured as the ratio of (Tet R + Amp R )/Amp R colonies. T1 and T2 are E.coli rrnB transcription terminators, which terminate read-through transcription by E.coli RNA polymerase, but not phage T7 RNA polymerase. Tf is a phage T7 transcription terminator. (B) Mobility frequencies. The target site is indicated to the left with (LEAD) or (LAG) indicating whether the site is located on the leading-or lagging-template strand of the E.coli chromosome. Mobility frequencies (%) are the mean -SD for at least three replicate experiments for each targetron/ target site combination. than predicted by the current computer algorithm (10), including sites used preferentially in single-stranded DNA and without second-strand cleavage. Finally, our findings indicate that En + -group II introns can switch surprisingly readily to Enindependent retrohoming pathways, thereby facilitating their dispersal to new target sites. Such a switch may in turn lead to dispensability of the En domain, possibly contributing to its loss multiple times during evolution in different group II intron lineages (28).