*Research School of Biological Sciences, The Australian National University, Canberra;
CSIRO Entomology, Canberra;
Department of Botany and Zoology, The Australian National University, Canberra
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Abstract |
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Introduction |
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Transposition of mariner-Tc1 TEs occurs by a cut and paste mechanism and is mediated by the transposase's ability to recognize the ITRs (Plasterk, Izsvak, and Ivics 1999
). The two functions of the transposase, sequence-specific DNA binding and excision and insertion of the TE, are mediated by separate DNA-binding and catalytic domains. At the N-terminus of the transposase is the predicted DNA binding domain, which for most mariner-Tc1 elements, is about 150 amino acids. For one member of the superfamily, Tc3, the crystal structure of the N-terminus of the DNA-bound transposase has been determined (van Pouderoyen et al. 1997
) and was observed to possess a DNA-binding domain containing a helix-turn-helix (HTH) motif like that of the DNA-binding paired-like domain of some transcription factors (Franz et al. 1994
; Ivics et al. 1996
). The HTH motif binds to specific ITR sequences of the Tc3 transposon. Although the overall amino acid identities are not high amongst the mariner-Tc1 transposases (30%; Robertson 1995
), secondary structure predictions and sequence alignments suggest that other transposases within the superfamily possess similar paired-like HTH motifs (Pietrokovsky and Henikoff 1997
; van Pouderoyen et al. 1997
; Plasterk, Izsvak, and Ivics 1999
). Similar analyses also predict the presence of a second HTH motif that resembles a homeodomain-like DNA-binding domain, and through DNase footprints and methylation interference studies, this domain has been shown to bind to ITR sequences adjacent to the sequences bound to the paired-like HTH (Vos, van Luenen, and Plasterk 1993
; Colloms, van Luenen, and Plasterk 1994
; Vos and Plasterk 1994
).
The second functional domain of the transposase is the (130 amino acid) catalytic domain, which is responsible for the site-specific cleavage and joining of the transposition process. The presumptive active site within this domain is defined by a three amino acid motif, consisting of two aspartic acid residues (D) separated by more than 90 residues in the primary sequence, followed by an aspartic or glutamic acid residue (E) at a typical distance of 34 or 35 residues (Doak et al. 1994
). The catalytic domain of mariner-Tc1 transposases displays distant DNA sequence similarity (39%45%) to several prokaryotic IS transposases and to some long terminal repeat (LTR) retroelement and retroviral integrases (Fayet et al. 1990
; Khan et al. 1991
; Doak et al. 1994
). The similarities in sequence and predicted structure across the superfamily may reflect functional conservation amongst the mariner-Tc1 TEs because similar mobility mechanisms have been observed for Tc1 (Vos, De Baere, and Plasterk 1996
) and three other members of the mariner-Tc1 superfamily, Tc3 (van Luenen, Colloms, and Plasterk 1994
), Mos1 (Tosi and Beverley 2000
; Zhang, Dawson, and Finnegan 2001
), and Himar1 (Lampe, Churchill, and Robertson 1996
). The elements are all thought to excise by double-stranded DNA breaks at the end of the ITRs. When repaired, the staggered catalytic cleavage at the 5' end of the transposon donor site leaves behind a characteristic footprint (Vos and Plasterk 1994
). Excision results in a copy of the element that can subsequently reintegrate elsewhere in the genome at a TA dinucleotide target site, and the integration is accompanied by duplication of the TA target on either end of the inserted element.
Although members of the mariner-Tc1 superfamily have several features in common, there are also features that distinguish mariner from Tc1 elements. Mariner elements are generally about 1.3 kb long, whereas Tc1 elements are slightly larger, ranging between 1.6 and 1.7 kb in length. The difference in length is often due to different lengths of the ITRs because the ITRs of mariner are about 30 bp, whereas those of Tc1 range from 20 to 460 bp. The nucleotide sequences of the ITRs also differ and there are some nucleotides that are characteristically conserved within each family (Robertson 1995
). Robertson (1995)
compiled consensus sequences for mariner and Tc1 transposases and identified 99 mariner residues and 86 Tc1 residues that can serve as distinguishing characters for each family. Mutation of some of these conserved residues has been shown to have a profound effect on transposition rates of the Mos1 mariner element in Drosophila melanogaster (Lohe, De Aguiar, and Hartl 1997
). However, the character which is used most often to distinguish between mariner and Tc1 is the catalytic triad motifmariner elements have a D,D,D catalytic triad for the transposase, whereas Tc1 elements have a D,D,E catalytic triad (Robertson and Asplund 1996
; Plasterk, Izsvak, and Ivics 1999
).
This article identifies a novel group of TEs that clearly belong to the mariner-Tc1 superfamily. These TEs have features intermediate to both mariner and Tc1 elements, and hence we have named them maT elements. We first detected a maT element as a repetitive sequence in the genome of the housefly Musca domestica, flanking and imbedded in the MdE7 gene implicated in organophosphate (OP) insecticide resistance (Claudianos, Russell, and Oakeshott 1999
). A GenBank search revealed sequences highly similar to the housefly sequence in the genome of C. elegans. Full-length maT elements from C. elegans were subsequently identified, and this article describes their distribution within the C. elegans genome and their phylogenetic relationship to other members of the mariner-Tc1 superfamily.
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Materials and Methods |
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Claudianos, Russell, and Oakeshott (1999)
described the isolation of clone
19.3.1 from a
Dash (Stratagene) genomic library made from adults of the Rutgers strain kindly provided by R. Feyereisen, University of Arizona, USA. Clone
19.3.1 contains an esterase gene (Md
E7) implicated in OP resistance. In the current study, DNA from
19.3.1 was subcloned into compatibly digested and prepared pBluescript SK(-) (Stratagene) plasmid vector with T4 DNA ligase (New England Biolabs). Nested deletions of subcloned EcoRI fragments were created using the Erase-a-Base system (Promega). Subcloned restriction fragments and deletion clones were sequenced on both strands using flanking primers (T3 and T7) and TaqFS dye-terminators. Reactions were analyzed using a 373A automated DNA sequencer (Applied Biosystems). In total, 10 kb of the 17.8 kb of
19.3.1 were sequenced. This included two novel
900 nt sequences associated with intron I and the 3' flanking regions of Md
E7. These sequences had significant sequence identity to each other (85%) and were named MdmaT1.A and MdmaT1.B, respectively.
Two primers, AE7.25 (5'-CGCAACAGAAAGAAAATAAAC-3') and AE7.26 (5'-ATCGACACTTTGGTATTTTT-3'), were used in PCR experiments to amplify the maT sequences from M. domestica genomic DNA and cDNA. The MdE7-specific primers that were used to amplify adjacent sequences from genomic DNA were: AE7.4 (5'-TCGATTATTTGGGTTTCATTTGT-3'), AE7.12 (5'-GGCATGGAAAACCTCACCTGG-3'), and AE7.30 (5'-ATGAATTTCAAAGTTAGTCAA-3'). PCR was conducted in a Corbett Research FTS-1 thermal cycler. The 50-µl PCR reaction mix contained 100 µg DNA, 50 pmol of each primer, 10 mM Tris-HCl (pH 8.3), 1.5 mM KCl, 0.25 mM of each dNTP, and 1 unit of Taq polymerase (GIBCOBRL). Amplification began with an initial denaturation step of 95°C for 3 min, followed by addition of the Taq polymerase at 80°C, and then 35 cycles of 1 min at 95°C, 1 min at 55°C, and 2 min at 72°C. A final 5 min at 72°C was used to fully extend all PCR products. PCR products were resolved in agarose gels and purified using a Qiagen QIAquick PCR purification kit according to the manufacturer's instructions. Purified amplicons were cloned into EcoRV-cleaved, T-tailed pGEM-T plasmid vector (Promega). Plasmid DNA was isolated (Wizard, Promega) and the DNA was sequenced using primers complementary to the T7 and SP6 promoters in the vector. DNA was sequenced using TaqFS dye-terminator chemistry (Applied BioSystems) on the Applied BioSystems Model 373A automated DNA sequencer.
Southern Analysis
Southern blot analysis of genomic DNA was carried out after separation of 10 µg of digested DNA on duplicate 1.0% agarose gels and blotting onto supported nitrocellulose membranes (NitroPure, Micron Separations Inc.). A 536-bp MdmaT1.A genomic PCR product was labeled by the random primer method and used as a probe. Prepared membranes were hybridized with 32P-labeled DNA at 42°C in a 50% formamide hybridization solution using standard techniques. High stringency blot washes were performed at 65°C in 2x SSC, 0.1% SDS, and low stringency washes were carried out using the same solutions at 50°C.
Phylogenetic Analysis, Sequence Alignment, and Molecular Modeling
The two M. domestica maT sequences were compared with nonredundant databases using the NCBI server with tblastx and tblastn (www.ncbi.nlm.gov/cgi-bin/BLAST). maT sequences were similarly identified from the completed C. elegans genome project (www.sanger.ac.uk/Projects/C_elegans/blast_server.shtml). Additional Tc1-, Tc3-, and mariner-like sequences within the C. elegans genome were also identified by comparison with sequences previously reported (Robertson 1995
; Robertson and Asplund 1996
; Gomulski et al. 2001
; Shao and Tu 2001
). Inferred translation products were used to estimate identity and distance scores. Pairwise comparisons were performed using the GCG alignment program "Gap" (Devereux, Haeberli, and Smithies 1984
), and multiple sequence comparisons and consensus sequences were generated using either the GCG program "Pileup" (Devereux, Haeberli, and Smithies 1984
) or "CLUSTAL W" (Thompson, Higgins, and Gibson 1994
) with default parameters (gapweight 5.0, gap length weight 0.3) for nucleotide sequences and the default scoring matrix (gapweight 3.0, gap length weight 0.1, and end gap penalties enforced for pileup; gap opening 10.0, gap extension penalty 0.1 for CLUSTAL W) for proteins. In multiple aligned sequences, the locations of indels (insertions and deletions) were adjusted as necessary so that they fell outside known structural elements of other transposon and paired-box proteins. Phylogenetic analyses were performed using PAUP* (Swofford 2000
) and PHYLIP program packages (Felsenstein 1989
). Distance trees were created using the neighbor-joining method with standard distances and mean character differences (PAUP*) and with "Prodist" using the "Categories-chemical model" (PHYLIP). Parsimony trees were created using simple heuristic search with fast stepwise addition (PAUP) and "Protopars" (PHYLIP). Confidence values were obtained by random resampling with 1,000 bootstrap replications.
Molecular modeling was performed using the EMBL Predict Protein server (Rost 1996
) and MolMod mirror site (Molecular Modeling and Structure Prediction at ANGIS). The analyses included PHDsec, PRO-SITE search, ProDom search, COILS algorithms, and incremental threading optimization onto known (3D) structures using TITO (Labesse and Mornon 1998
). PSORT II analysis (Horton and Nakai 1997
) was used to predict for the presence of nuclear localization signals (NLSs).
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Results and Discussion |
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A combination of MdmaT (AE7.25) and MdE7 esterase (AE7.4, AE7.12) primers were designed to identify and characterize MdmaT1.A and MdmaT1.B from the sbo fly strain. The primer pair AE7.12 and AE7.25 produced a 3.1-kb PCR product and confirmed that MdmaT1.B and the adjacent Shaw sequence are present at the 3' end of Md
E7 in both the OP-resistant Rutgers and -susceptible sbo strains (results not shown). However, MdmaT1.A was not detected in sbo flies using AE7.25 and AE7.4 or any other combination of primers. Southern blot analyses of genomic DNA from various housefly strains probed with the full-length Md
E7 cDNA indicate significant differences at the 5' end of the Md
E7 gene among strains (Claudianos 1999
).
An attempt was made to PCR amplify MdmaT sequences from housefly cDNA. PCR experiments using a third instar Rutgers Uni-Zap cDNA library as a template and the primer pair AE7.25 and AE7.26 consistently produced 500 bp products. The products were cloned and sequenced. Two similar sequences of 497 and 498 bp encode 99% identical peptides and contain a single common stop mutation (amino acid position 129), resulting in truncated ORFs (GenBank accessions AF324221 and AF324222). A consensus housefly MdmaT sequence was generated based on MdmaT1.A and MdmaT1.B and the two cDNA sequences. The resulting 1,050 bp sequence encodes a 350 amino acid protein that has 35% identity and 55% similarity to the 346 amino acid consensus sequence of the Bmmar1 transposon (fig. 2A ).
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maT in C. elegans
Examination of the C. elegans genome sequencing project (www.sanger.ac.uk/Projects/) revealed a total of 16 maT sequences apparently randomly distributed throughout the nematode's genome (table 1
). Twelve of the 16 C. elegans maT sequences, all denoted CemaT1, are highly similar, with greater than 99% identity. Four of these 12 CemaT1 elements contain a small number (15) of coding, frameshift and stop mutations, and are likely nonfunctional (table 1
). The remaining eight CemaT1 copies contain a single 1,008 nt ORF encoding a 336 amino acid peptide. These apparently intact CemaT1 elements have perfect 26 nucleotide palindromic ITRs and are flanked by TA dinucleotide insertion duplications. When viewed together, the relatively small number of copies, the high ratio of putative functional to nonfunctional sequences, and the limited sequence divergence suggest that the invasion of CemaT1 into the genome of the C. elegans N2 strain genome was a relatively recent event.
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A pairwise sequence comparison of the consensus housefly MdmaT and nematode CemaT1 encoded proteins (GenBank accession U41268, nt 7,2648,271) shows 38% identity and 58% similarity (fig. 2A
). The consensus MdmaT has 29% and 36% identity to CemaT2 and CemaT3, respectively. A multiple alignment of CemaT1 and Bmmar1 encoded proteins verifies a common C-terminal D,D,D catalytic triad characteristic of mariner transposase proteins (Doak et al. 1994
). Similarly, the maT encoded protein has N-terminal similarity to the DNA binding and DNA recognition domains of Tc1 and Tc3 transposons (Colloms, van Luenen, and Plasterk 1994
). A schematic of the sequence organization and putative protein characteristics of CemaT1 is shown in figure 2B.
maT as a Novel Clade
A multiple alignment of maT, Tc1- and mariner-related proteins as well as a distantly related bacterial transposon was used to construct phylogenetic trees (fig. 3A and B
). These evolutionary analyses confirmed that Bmmar1, MdmaT, and CemaT sequences cluster together as a single clade within the mariner-Tc1 superfamily. A number of recently identified transposons (Shao and Tu 2001
) of the mariner-Tc1 superfamily, two from the nematode Caenorhabditis briggsae (C.briggsae.ITmD37D1 and C.briggsae.ITmD37D2), and one from the dipteran insect Sarcophaga peregrina (S.peregrina.ITmD37D1) also fall within this clade. Two somewhat related mosquito sequences (An. gambiae and Ae. atropalpus ItmD37E1; Shao and Tu 2001
) show a statistically supported monophyletic relationship with maT elements (fig. 3
). Intriguingly, the apparent close phylogenetic relationship of the mosquito DD37E elements to DD37D maT elements conflicts with the nominal catalytic motif (D,D,D/E) classification system.
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A multiple alignment of experimentally determined or putatively identified active members of the mariner-Tc1 superfamily was used to construct maximum parsimony trees for both the DNA binding and catalytic domains of the transposase protein (fig. 4A and B,
respectively). The phylogenetic analyses of the two separate domains generated trees with similar topology to each other as well as to the previous extensive phylogenetic analyses based on the total transposase sequence (fig. 3
). Although maT transposases have a D,D,D catalytic triad, which has often been used as a defining character for mariner elements, phylogenetic analysis of the catalytic domain still predicts a closer relationship between Tc and maT elements than mariner and maT TEs. It seems unlikely that maT elements are hybrid TEs that resulted from a recombination between a Tc and a mariner element because both of maT's transposase domains appear to be more Tc-like than mariner-like. Interelement recombinations or gene conversions are considered to have given rise to various hybrid elements, including LINE retrotransposons in humans (Saxton and Martin 1998
), Ty retrotransposons in yeast (Jordan and McDonald 1998
), and numerous bacterial (insertion sequences, IS) transposons (Mahillon and Chandler 1998
). In all these cases however, phylogenetic analyses suggested that the putative hybrid elements had acquired their two functional domains from different elements. In contrast, there is no evidence to suggest that maT elements acquired one of their two transposase domains from a mariner element and the other from a Tc element.
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Molecular Modeling of maT: Structure-Function Analyses
The 130 amino acid N-terminal domain of C. elegans maT1 transposase has 23% identity and 42% similarity to the N-terminal DNA-binding domain of the C. elegans Tc3 transposase (Tc3A; Collins, Forbes, and Anderson 1989
) and 24% identity and 52% similarity to the DNA-binding paired domain of the Pax-paired (prd) family of transcription factors of Drosophila (Bopp et al. 1986
) and mammals (Franz et al. 1994
). The corresponding N-terminal domain of the mariner element Mos1 has less identity (20%) to CemaT1 than either Tc3A or prd. Secondary structure predictions of the CemaT transposase N-terminal domain (fig. 5
) identified six putative
-helices (residues 914, 1824, 3244, 6268, 8088, 109124) in an HTH arrangement typical of eukaryotic paired and homeobox proteins, as well as prokaryotic
repressors and Hin recombinase proteins (Wintjens and Rooman 1996
).
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The alignment of the DNA-binding domains of CemaT1, Tc3, Mos1, and prd shows that the first three helices of the CemaT1 transposase closely match in position and size the equivalent helices from the DNA-binding subdomains of Tc3A-N and prd (fig. 5
) In contrast, the mariner element Mos1 differs from the others because it shares only two of the three helices in this subdomain, and two of these helices are separated by a longer intervening loop than that observed or predicted for the other sequences examined. X-ray crystallography data for the N-terminal domains of Tc3A and prd indicated that the two proteins share highly similar topographies, with only 2.3 Å difference between the DNA-binding domains of the two proteins and 0.66 Å difference between the connecting loops of the Tc3A-N and prd structures (van Pouderoyen et al. 1997
). Threading optimization of the CemaT1 transposase N-terminus with prd and Tc3 (using TITO software) predicted a hypothetical structure of CemaT1 that closely matched the three-dimensional structures of both prd and Tc3A-N, falling within the 2.3 Å helix and 0.66 Å loop tolerances suggested by the crystal structure comparison of Tc3A-N and prd (results not shown).
Atomic structure comparisons show Tc3A shares 11 of the 16 sugar-phosphate DNA contacts of prd, and four of these are identical residues (residues 15, 46, 51, and 70 of prd). The relative positions of the HTH motifs involved in base-specific major and minor groove (DNA-helix) docking interactions are also conserved between these two proteins (fig. 5
). CemaT1 shares seven and four identical DNA contact residues with prd and Tc3A, respectively. In contrast, Mos1 shares only one identical contact residue with either prd or Tc3A. The relative lack of conservation of nucleotide-specific docking residues makes it difficult to predict any maT-specific target sequences. However, like Tc3A and Tc1A (Colloms, van Luenen, and Plasterk 1994
; van Pouderoyen et al. 1997
), these nucleotides are expected to be within the 26-bp ITR, and would likely differ between Tc3 and CemaT1, because their ITR sequences differ by 54%.
PHDsec predictions also suggest that the CemaT1 transposase has the equivalent of the prd and Tc3A helices 4, 5, and 6, along with an additional helix predicted at residues 93100, between helices 5 and 6. A GRPR-like sequence is conserved in mariner-Tc1 transposases between helices 3 and 4, and CemaT1 similarly has a GRPP sequence. This motif is characteristic of homeodomain proteins (Gehring et al. 1994
) and mediates DNA interactions of these and related proteins. Overall, the primary sequence and secondary structure profile of the N-terminus of the CemaT1 protein suggest it is more closely related to the prd protein than to either the Tc3A-N or Mos1 peptides. However, important differences clearly exist in the DNA-binding domains of the prd, Tc3A-N, Mos-1, and CemaT1 transposase. The few residues preceding the first helix of Tc3A adopt a conformation different to that for the longer N-terminus of the paired domain, which forms a small ß-sheet structure (Xu et al. 1995
; van Pouderoyen et al. 1997
). In this respect, CemaT1 transposase appears to be more similar to Tc3A and Mos1 and may have the same ability to affect the conformational changes to DNA needed for transposon-transposase complementarity.
The mariner-Tc1 TE proteins contain a bipartite-type NLS comprising two basic amino acids followed by a 10 amino acid spacer and a cluster of three basic residues (Ivics et al. 1996
). PSORT II analysis of the proteins shows that the last helix of CemaT1 and Mos1 DNA recognition domains overlaps the bipartite NLS (fig. 5
). Although the last helix of CemaT1 aligns reasonably well with that of Tc3, the last helix of Mos1 does not align with the positions of the last helices of any of the other sequences examined due to the presence of additional intervening sequence between the last two helices.
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Conclusions |
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Acknowledgements |
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Footnotes |
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Keywords: transposon
mariner
Tc1
Caenorhabditis elegans
Musca domestica
MdE7
Address for correspondence and reprints: Charles Claudianos, Research School of Biological Sciences, The Australian National University, G.P.O. Box 475, Canberra, ACT 2601, Canberra. E-mail: claudianos{at}rsbs.anu.edu.au
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