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DINE-1, the highest copy number repeats in Drosophila melanogaster are non-autonomous endonuclease-encoding rolling-circle transposable elements (Helentrons)
Mobile DNAvolume 5, Article number: 18 (2014)
The Drosophila INterspersed Elements-1 (DINE-1/INE1) transposable elements (TEs) are the most abundant component of the Drosophila melanogaster genome and have been associated with functional gene duplications. DINE-1 TEs do not encode any proteins (non-autonomous) thus are moved by autonomous partners. The identity of the autonomous partners has been a mystery. They have been allied to Helitrons (rolling-circle transposons), MITEs (DNA transposons), and non-LTR retrotransposons by different authors.
We report multiple lines of bioinformatic evidence that illustrate the relationship of DINE-1 like TEs to endonuclease-encoding rolling-circle TEs (Helentrons). The structural features of Helentrons are described, which resemble the organization of the non-autonomous partners, but differ significantly from canonical Helitrons. In addition to the presence of an endonuclease domain fused to the Rep/Helicase protein, Helentrons have distinct structural features. Evidence is presented that illustrates that Helentrons are widely distributed in invertebrate, fish, and fungal genomes. We describe an intermediate family from the Phytophthora infestans genome that phylogenetically groups with Helentrons but that displays Helitron structure. In addition, evidence is presented that Helentrons can capture gene fragments in a pattern reminiscent of canonical Helitrons.
We illustrate the relationship of DINE-1 and related TE families to autonomous partners, the Helentrons. These findings will allow their proper classification and enable a more accurate understanding of the contribution of rolling-circle transposition to the birth of new genes, gene networks, and genome composition.
Repetitive DNA constitutes a major portion of most multicellular eukaryotic genomes. This fraction includes tandem and interspersed repeats. Transposable elements (TEs) are the major constituent of the interspersed repetitive DNA. Class 1 retrotransposons utilize an RNA intermediate and Class 2 DNA transposons utilize a DNA intermediate as the basis for transposition (for review ). The RNA mediated reactions are a copy-and-paste mechanism because an RNA transcript is copied to DNA. The cut-and-paste DNA transposons move the excised double-stranded DNA sequence of the transposon to a new target location (for review ). The rolling-circle transposons (Helitrons) like the retrotransposons use a copy-and-paste mechanism despite moving a DNA intermediate . They are hypothesized to mobilize a single-stranded DNA molecule to a new target location . Whether a TE family uses a copy-and-paste versus a cut-and-paste mechanism in part influences its relative abundance in a genome (for review ). TEs are distinguished from other forms of repetitive DNA because they replicate via self-encoded proteins (autonomous) or by hijacking TE encoded proteins (non-autonomous) (for review ). Hijacking the proteins of autonomous TEs appears to be a successful strategy as non-autonomous TEs generally outnumber the autonomous partners. Therefore both the replication mechanism, copy versus cut-and-paste and whether or not the TE is protein-coding influence copy number.
Genome-wide analysis allows the identification of entire populations of TEs in a genome, which includes old and inactive families as well as young, active copies. The classification of autonomous TEs is generally straightforward based on the encoded protein(s), the structure of the mobile unit, and the modification of the target site (TSD). While, the non-autonomous TEs do not encode proteins, they can often be classified based on structural features and TSD. Indeed, non-autonomous TE families can be linked to autonomous partners because they share common structure and TSD, which greatly facilitates classification. However, some non-autonomous families defy classification because they lack distinctive structural features, which enable a link to be made to autonomous partners.
The Drosophila Interspersed elements (DINE-1) identified  on the dot chromosome and heterochromatic regions in the D. melanogaster genome fall into the category of non-autonomous TEs that have defied classification [6, 7]. While DINE-1 elements lack coding capacity they do display well-defined structural features. The structural features include 13 bp subTIRs, a short inverted repeat (IR) (approximately 3 to 22 bp away from the 5’ subTIR) and a short stem loop at the 3’ end  (Figure 1). Analyses of target site preferences in drosophilid genomes revealed a clear insertion preference for a TT dinucleotide but were inconclusive as far as target site modification [5–7]. These analyses allowed the definition of a core DINE-1 sequence and the observation that they carry a short microsatellite sequence . DINE-1 elements have also been called INE-1 and DNAREP1 . For the purposes of this study we refer to all related families from Drosophila as DINE-1.
TEs with structural characteristics similar to DINE-1 (DINE-1 like) have been described in the several other genomes including several drosophilids (Additional file 1: Table S1) [10–20]. DINE-1 like elements are most abundant in the centromeres and heterochromatic regions of the chromosomes of drosophilids [5, 6, 10, 16, 21]. In D. serido they also predominate the heterochromatic regions of the sex chromosomes . Recently active copies were found more evenly spread towards the chromosome arms and in the euchromatic regions [6, 7, 10]. Some DINE-1 like elements contain tandem repeats [16, 22–24] in addition to the microsatellites that are typical of the structure .
Outside of Drosophila, DINE-1 like elements have been identified in many lepidopteran [25–27], hemipteran , and dipteran [10, 28] genomes (Additional file 1: Table S1). These elements are also described from the genomes of sea urchin  and molluscs (Additional file 1: Table S1) [22, 23, 29]. DINE-1 like elements have attained high copy number in drosophilids  and other insect and invertebrate genomes (Additional file 1: Table S1) [22, 24, 25, 27, 28]. In D. melanogaster DINE-1 elements are the most abundant TE (for review ) and constitute 9.2% (approximately 114 kbp) of the dot chromosome .
DINE-1 like elements are reported to be involved in the duplication or generation of novel genes in Drosophila although whether this is direct or indirect is unclear and the mechanism remains elusive. Most of the known functional duplicates are associated with gametogenesis [31–34]. In D. miranda the DINE-1 like elements provide the binding sites for male specific lethal complex, which regulates dosage compensation [12, 35]. In addition, DINE-1 elements are reported to provide putative transcription factor binding sites in insecticide resistance associated Cyp genes . The myriad of ways that DINE-1 elements have impacted Drosophila evolution [12, 31–34, 36], suggests many important innovations may await discovery.
The lack of structural similarity with an autonomous partner precluded the proper classification of DINE-1 families. Indeed, these elements were initially classified as non-LTR retrotransposons perhaps because of the asymmetrical ends [5, 9, 11, 14]. The interpretation of TSD creation led to the classification as the non-autonomous partners of classic cut-and-paste DNA transposons or Miniature Inverted-repeat Transposable Elements (MITEs) in some cases [6, 10, 22, 23, 26, 27]. However, in other examples, no TSD could be readily identified and these elements were classified as Helitrons. Further support of the Helitron classification came with the identification of gene fragments with homology to the proteins encoded by some Helitrons, Helentrons and non-LTR retrotransposons [7, 25, 37] (for review ). Because the structure of the DINE-1 like elements differed from canonical Helitrons, the relationship to Helitrons was not clear .
The canonical Helitrons have well defined ends (5' TC and a 3' CTRR) as well as a 16 to 20 nucleotide palindrome, which is approximately 11 bp away from the 3' end . They always insert between A and T nucleotides and do not create any TSD . A putatively autonomous animal Helitron typically encodes a single transposase open reading frame (ORF) (Rep/Helicase) with a zinc-finger, a rolling-circle motif (Rep), and a helicase domain. The plant-Helitrons encode additional ORFs related to ssDNA binding protein, Replication Protein A (RPA) (for review ). The non-autonomous partners of Helitrons vary in length, but share structural homology with the autonomous Helitron partners.
Recently two subtypes of Helitrons have been identified, Helentrons and Helitron2. Helentrons are so called because of the presence of an apurinic/apyrimidinic (AP) endonuclease domain fused to the C-terminus of the Rep/Helicase protein . Helentrons encode additional ORFs with homology to OTU cysteine proteases [40, 41] and RPA proteins . The structural features of Helentrons are not known despite reports describing the coding capacity [38, 40, 41]. The Helitron2 elements have asymmetrical terminal inverted repeats and palindromic sequences on both ends. These elements encode a single ORF corresponding to the Rep/Helicase protein with no endonuclease domain . However, the relationship between Helitrons, Helentrons, and Helitron2 elements is not well understood.
Here we describe for the first time the structural features of Helentrons and describe their relationship to Helitrons and Helitron2 elements. This analysis has allowed us to unequivocally link Helentrons to their non-autonomous partners (the DINE-1 like). Previously reported genomic impacts of DINE-1 like transposons are discussed in light of the rolling-circle transposition mechanism. This includes the involvement of DINE-1 elements in gene duplication by de novo chimeric gene assembly and the structural features that predispose co-option into regulatory networks. Presented is a new classification scheme for Helentrons and DINE-1 like families and subfamilies that take into account their sequence heterogeneity. A model of the relationship of Helentrons to Helitrons and the intermediates identified in some organisms is presented.
Identification and characterization of DINE-1 like elements in the mite genome
A TE survey of the genome of the western predatory mite, Metaseiulus occidentalis lead to the identification of interspersed repeat families with defined boundaries that lack coding capacity (HINE-Mo-1-7). These repeats are characterized by 12 to 15 bp subterminal palindromic inverted repeats (subTIRs) and a 5 to 10 bp palindrome near the 3' end (Figure 1A-B). The subTIRs are approximately two to four nucleotides away from the 5' termini and approximately 38-60 bp away from the 3' termini. The palindromes are approximately eight to 35 bp of the 3’ termini (Figure 1A-B). Some copies contain microsatellites or tandem repeats. They display an insertion preference for T-rich sequence.
HINE-Mo-(1-7) repeats share most of the structural features of the DINE-1 elements described from Drosophila, which includes the presence of subTIRs and palindromes on both ends (Figure 1C) [5–7]. Although the HINE-Mo subTIRs are themselves palindromic, a pattern not observed for the DINE-1 elements (Figure 1B, C) . The HINE families differ from DINE-1 in that they do not have the IR and core sequence (Figure 1C) . However, the common structural characteristics suggest that both HINE and DINE-1 belong to the same superfamily and utilize a similar mechanism of transposition.
Target site modifications
To confirm the boundary of HINE-Mo insertions, empty sites (insertion free sites) were identified (see Methods). We carefully analyzed multiple sites for each family and did not find any evidence of target site modification. HINE elements preferentially target a T-rich sequence and each insertion is flanked by at least two T nucleotides on each end, although it is unclear if they are distributed symmetrically (two TT on each termini) or asymmetrically (Figure 2A-C). Interestingly, in some cases elements of the same family differ in the number of T nucleotides on the termini, as evident from the analysis of paralogous empty sites (Figure 2D, E, Additional file 2: Figure S1, Additional file 3: Figure S2). Because these elements target T rich sequence, it is unclear if the Ts are distributed to only one end or both ends (Additional file 3: Figure S2). In sum, we did not find any evidence of target site modifications triggered by HINE integration, which is consistent with a single-stranded replication intermediate .
Autonomous partner and the coding capacity
We employed homology-based searches to identify the autonomous partners of the HINE families. Repeats could be identified that share significant sequence identity (84% to 99.9%) with two subfamilies HINE-Mo-1A (Figure 3A-C) and HINE-Mo-1 K (Figure 3D-G). Pairwise alignments revealed that HINE elements are simple deletion derivatives of longer novel Helitron- like repeats that share the structural features displayed by the HINEs (Figure 3).
To determine the coding potential of the Helitron-like elements, we employed both conceptual translation of putative ORFs and homology-based searches (Blastx/conserved domain database (CDD))  to the protein database. The CDD searches  and Blastx were employed to identify putative functional domains of the Helentron encoded proteins and related sequences in other organisms. The Helitron-like element from the mite encodes two putative proteins, which is atypical of animal Helitrons (that encode a single Rep/Helicase protein). The first corrected ORF (frame +1; no introns; 1,881 aa) shares 42% similarity over 1,336 aa with the Rep/Helicase/Endonuclease protein (Accession: DAA01284.1) identified from Danio rerio. The alignment spanned the C-terminal endonuclease domain, which is a distinctive feature of Helentrons and is not associated with canonical animal Helitrons. The closest hit was to a putative hypothetical protein identified in the Culex quinquefasciatus genome (Accession: XP_001864092.1) (61% similarity over 1,747 aa).
Zn-finger-like motifs [38, 40] are identified in the N terminal region of the predicted protein. The mite Rep motif shares significant sequence similarity with that of diverse Helitrons (Helitron-like, E-value: 1.62e-19, Cdd:pfam14214) , Helentrons, and the replication protein encoded by viruses and plasmids that utilize rolling-circle replication (RCR) (Additional file 4: Figure S3). The three conserved motifs that are necessary for the RCR [2, 38, 40, 43, 44] (for review ) are conserved in the mite Helitron-like element (Additional file 4: Figure S3). The helicase domain identified downstream of the Rep motif has the strongest similarity with the PIF1 family belonging to the super family 1 (SF1) of helicases (PIF1-like helicase, E-value: 6.90e-19, Cdd:pfam5970) . Alignment of the eight motifs that typify the PIF1 helicase family are conserved in the both the Helentrons and Helitrons (Additional file 5: Figure S4) . Fused to the C-terminus of Helicase within the same ORF, the apurinic/apyrimidinic (AP) endonuclease was identified (Exo_Endo_phos_2, E-value: 1.25e-06, Cdd:pfam14529)  as previously described for Helentrons. The endonuclease of the mite Helitron-like element displays the characteristic seven domains described from cellular AP endonucleases, Helentrons and the non-LTR retrotransposon encoded protein (Additional file 6: Figure S5) [38, 45].
The second corrected ORF (+3; no introns; 358 aa) putatively encodes the 70 kDa subunit of the single strand (ss) binding, Replication Protein A (RPA). The CDD searches revealed homology to the DNA binding Domains A (RPA1_DBD_A, E-value: 2.02e-15, Cdd:cd04474) and B (RPA1_DBD_B, E-value: 2.36e-12, Cdd:cd04475)  of RPA. Therefore the mite Helitron-like elements display all of the protein domains typical of Helentrons rather than Helitrons.
To further confirm that the mite Helitron-like elements belong to the Helentron group rather than Helitron group, a phylogenetic analysis (Neighbor-Joining (NJ) and maximum likelihood (ML)) was employed using an amino acid alignment of the Rep motif and eight helicase domains . In this phylogenetic analysis, Helentrons (including the mite Helentron) and Helitrons form separate clades (Helentron clade bootstrap score of 95 NJ/54 ML; Helitron clade bootstrap score of 100 NJ/100ML) (Additional file 4: Figure S3B). Therefore the phylogenetic analysis provides an additional line of evidence that non-autonomous families from the mite genome are Helentrons. We call the autonomous elements, Helentron-Mo. Hence the non-autonomous partners are called Helentron-associated INterspersed Elements, in short as HINEs. It is interesting to note that the Rep alignment revealed diagnostic amino acid positions that in most cases can be used to quickly distinguish between proteins encoded by Helentrons and Helitrons (Additional file 4: Figure S3A). The diagnostic positions include three amino acids (F/Y w/l/y/k R) near the motif 2 ‘(V/I)ExQxRG(S/L)(P/L)HxH’ which distinguish Helentron protein from Helitron protein. In addition, the three amino acids directly preceding the well-conserved histidine residues reveal another diagnostic site. The Helentrons have an ‘S’ amino acid between the G and P while the Helitrons have an L amino acid (Additional file 4: Figure S3A). These two signatures are sufficient to distinguish Helentrons from Helitrons in our sample.
Association of Helentrons and HINEs in other organisms
To determine if related Helentrons were present in the organisms where DINE-1 like families had already been described or vice versa, full-length Helentrons and non-autonomous families were mined from representative species (see Methods). Two families of Helentron and their derived HINEs and a novel family of HINE (Figure 4A-D, Additional file 7: Table S2, Additional file 8) were mined from the C. quinquefasciatus genome. The Culex Helentrons display similar structural characteristics as that of the mite including the palindromic subTIRs and the stem loop at the 3' end (Additional file 7: Table S2). The elements preferentially insert within the TT dinucleotide and have a string of Ts (2 to 5) on the boundaries (Additional file 2: Figure S1, Additional file 3: Figure S2). These Ts are part of the element but sometime vary in number between copies (Additional file 2: Figure S1, Additional file 3: Figure S2). The non-autonomous families share >95% sequence identity with the respective partner, Helentron. Some copies contain simple or tandem repeats that occupy approximately 50% of the total length of the element (HINE-Cq-32A.1 and HINE-Cq-31A.1).
We investigated the genomes that are known to harbor recently amplified DINE-1 s for the presence of the autonomous partner, Helentrons (D. ananassae, D. willistoni, and D. yakuba) . To this end, we identified the autonomous partners of the three different DINE-1 elements described in D. ananassae, D.willistoni, and D. yakuba (Additional file 8). The DINE-1 s are 98% identical with their partner Helentron and have similar structural characteristics including 13 bp subTIRs, IR, and a stem loop (Figure 4E, F, Additional file 7: Table S2). Interestingly, the Rep/Helicase/Endonuclease protein identified in the Helentron-Da-40 in D. ananassae is in the minus orientation as opposed to plus orientation found in the majority of these elements (Figure 4E). In addition, we have identified a novel family of Helentrons as well as a non-autonomous family in both D. willistoni and D. ananassae. This Helentron family has the palindromic subTIRs instead of the IR (Figure 4G-J). As observed earlier, these elements display an insertion preference for a TT dinucleotide, have variable number of Ts at their termini, and their boundaries were confirmed by identifying paralogous empty sites (Additional file 2: Figure S1, Additional file 3: Figure S2). The copy numbers are greater than the Helentron partner for all the non-autonomous Helentrons families across different organisms. Helentron proteins (not full-length elements) were identified in the genomes of Strongylocentrotus purpuratus, Bombyx mori, Danaus plexippus, Rhodnius prolixus, where DINE-1 like elements were previously reported although incorrectly annotated as MITEs [24–27] (Additional file 8). These data suggest that Helentrons frequently give rise to deletion derivative/non-autonomous families.
Classification of Helentrons and HINEs
The terminal sequences including subTIRs and stem loops are shared in both Helentrons and HINEs and hence might be the signature structures necessary for the transposition. In some cases, the subTIRs are (nearly) identical (Figures 3 and 4, Additional file 7: Table S2) between two Helentron families that share <70% identity at the nucleotide level within a species or across species (Figures 3 and 4). Due to the heterogeneity and peculiar structural features of Helitrons, a new classification criterion was established to define families and subfamilies . Because Helentrons have a different structure than Helitrons, the established classification scheme does not apply. To take into account the peculiar structural features of Helentrons in relation to Helitrons, we propose a new classification scheme to identify and classify different families of Helentrons. Helentrons containing at least 11 bp identical subTIRs are classified as members of a family (represented as the number in the name designation). Members of a subfamily share at least 80% identity over the last 60 bps of the 3' end (Figure 1) (represented as the letter following the number). The last 60 bps include the 3' subTIRs and stem loop. Using these criteria, seven families and 21 subfamilies of Helentrons and HINEs in the M. occidentalis genome (represented as Mo) were identified (Figure 1, Additional file 7: Table S2, Additional file 8). In addition, the mite genome harbors fragmented Helentrons (without ends) that diverged (>30%) from each other. This classification scheme for Helentrons will help to understand the diversity of these families within and across genomes.
Distribution of Helentrons
We employed a homology-based search to identify autonomous Helentrons in the sequences available at the whole genome shotgun (wgs), Nucleotide collection (nr/nt), Genome Survey Sequences (GSS), and High Through Genome Sequence (HTGS) databases. The Helentron protein queries derived by the conceptual translation of ORFs encoded by mite, Culex, Drosophila, fish, and fungi were used in Tblastn searches. The conserved amino acids of Motif two of the Rep were used as a proxy to differentiate the Helentron from Helitron proteins (Additional file 4: Figure S3). These analyses reveal the presence of Helentron proteins in many fish, Nematostella (Cnidaria), sea urchin (Echinodermata), and insects expanding the list of those previously reported (3 more insect orders (Coleoptera, Hymenoptera, Hemiptera)); 10 more families of fish; three more families from Cnidaria and Echinodermata groups (Additional file 8) [25, 38, 40, 41] (for review ) (Additional file 9: Table S3). In addition we report the presence of Helentron proteins in arachnids, fungi, nematodes, molluscs, rotifer, Cephalochordata, Hemichordata, Priapulida, Annelida, lampreys, and coelacanth (Additional file 8). We have identified Helentron proteins in a range of vertebrates and invertebrates that were not known before (Additional file 8). In addition, we identify Helentron proteins in the Cotesia sesamia Mombasa bracovirus (a virus integrated in the genome of some wasps) (for review ). Sequences that displayed homology to Helentron- Rep proteins were identified in green algae, red algae, and Oomycete genomes (Additional file 8). Interestingly, Helentrons are not identified in most plant genomes, but a few hits were found in the databases. Because the hits were to contigs with short length and that were low copy number, we could not rule out that they were contamination (Additional file 8). Overall, Helentrons have a broad distribution in Opisthokonts (for review ) although absent from mammals, but are otherwise very limited in their distribution taxonomically.
Identification of a Helitron-Helentron intermediate
In our survey of the Phytophthora infestans (Oomycete) genome, a Rep/Helicase protein (1,783 aa; +2, no introns) was identified that grouped phylogenetically with the Helentron Rep/Helicase group but did not have the endonuclease domain in the C terminus (Figure 5B). They have the signature amino acids in the Rep motif as observed with other Helentrons (Additional file 4: Figure S3). The structure of a full-length representative Helentron from P. infestans, which we call as proto-Helentron-Pi (approximately 14 kb) displayed the structural features more similar to Helitrons than Helentrons (5'TT and 3' CTAG) (Figure 5). The boundary of the element was confirmed by identifying a paralogous empty site (Additional file 2: Figure S1). None of the insertions are flanked by target site duplication (Figure 5, Additional file 2: Figure S1) and they insert between an A and T nucleotides like Helitrons (Additional file 2: Figure S1). Thus the Rep/Helicase protein is closest in amino acid similarity to Helentrons, yet the structural features of the complete elements are those of typical Helitrons (Figure 5). This suggests that Phyotophthora proto-Helentron could be an intermediate between Helitron and Helentron representing the element before the gain of the endonuclease domain.
The proto-Helentron-Pi encodes multiple putative proteins in addition to the Rep/Helicase protein. The first and last ORFs encode hypothetical proteins (+2; 218 aa; EEY70650.1 and -3; 266 aa; EEY64853.1). The second corrected ORF (approximately 153 aa, +3; no introns) encodes a putative SET domain (SET, E-value: 3.42e-24, Cdd:smart00317)  protein that has 50% similarity with the SET domain (121 aa) of histone H3 methyltransferase Clr4 of yeast, Schizosaccharomyces pombe (Accession: NP_595186.1). The characteristic features of the SET domain including the presence of variable insert region (SET-I) between the SET-N and SET-C regions (for review ) are present (Additional file 10: Figure S6). The highly conserved regions along with the well-conserved co-factor binding sites are present within the N and C regions (Additional file 10: Figure S6).
Gene capture by Helentrons
To determine if Helentrons capture gene fragments like Helitrons, we employed Blastx/CDD searches to identify any potential gene fragment within Helentrons and HINEs. Interestingly, Helentron-Cq-32A in the C. quinquefasciatus genome carries fragments of two histone genes (H4 superfamily E-value: 1.02e-06, Cdd:smart00428 and H15 superfamily E-value: 6.34e-05, Cdd:cd00073) . Eight copies carrying these gene fragments are present in the genome (Figure 4A). The H4 gene fragment within Helentron has 84% sequence identity over 244 bp with the putative parental in the Culex genome (AAWU01017064.1|:c10903-11310). Similarly, the H15 gene fragment has 83% identity over 87 bp with the putative parental in the Culex genome (AAWU01027066.1|:c14114-14203). However, both translated gene fragments contain a premature stop codon, frame shift, and indels when compared to the parental histone genes. The histone gene capture by a Helentron has not been documented in other organisms. In addition, the P. infestans proto-Helentron contains a fragment of a gene encoding a putative transmembrane protein (PITG_05761). The gene fragment within Helentron has 76% identity (1688 bp) with the putative parental gene in P. infestans. A short ORF (202 aa) is identified within the gene fragment in the Helentron, which contains the Ankyrin repeats (ANK, E-value: 5.20e-03, Cdd:cd00204) and the bacterial Toll-like Receptors (TIR_2) domain (TIR_2, E-value: 4.15e-15, Cdd: pfam13676) . The proto-Helentron-Pi is amplified to 25 copies in the P. infestans genome. We did not find any potential protein-coding gene fragment within the diverse mite Helentrons and HINEs (seven families and 21 subfamilies). However, these findings suggest that Helentrons can transduce and amplify gene fragments but the frequency of capture varies between genomes, like their Helitron relatives.
Helentrons are the autonomous partners of DINE-1 and related TEs
Our discovery of multiple autonomous Helentron families in the genome of the western predatory mite facilitated the identification of features that had not been previously defined but that are common to all endonuclease-encoding Helentrons. The structural features and target preference for T-rich sequence are shared with many non-autonomous TE families. These families known as DINE-1 like or HINEs are present in many genomes. Previously DINE-1 like elements have been misclassified as canonical Helitrons, MITEs, and non-LTR retrotransposons. Here we solve the mystery of DINE-1 classification by providing evidence that Helentrons are the autonomous partners (see Figures 3 and 4). Links were made between non-autonomous families and Helentrons in diverse genomes including mite, Culex, and D. ananassae, D. willistoni, and D. yakuba. Collectively, we call all non-autonomous Helentrons HINEs (for Helentron-associated interspersed elements).
HINE genomic impact linked to rolling-circle transposition mechanism
Movement by rolling-circle transposition may help explain some of the observations previously reported for HINE families, for example, high copy number in some genomes, the formation of clusters, and the association with gene duplicates. Helitrons make up a considerable fraction of the genomes of the vespertilionid bats , including the little brown bat (Myotis lucifugus) (Thomas et al. in revision GBE) and the lepidopteran, Heliconius melpomene genome (6% and 6.62%, respectively) . High genome content is consistent with a copy-and-paste transposition mechanism as is proposed for rolling-circle transposition. Clusters of Helitrons arranged in tandem arrays occur in the M. lucifugus genome  and B. mori and are thought to occur when a termination signal is bypassed. In the bat and maize genome, distantly related Helitrons are also found in close proximity to other Helitron insertions sometimes directly next to one another (Thomas et al. in revision GBE), . A similar pattern might be seen with Helentron mediated transposition events and might account for the clustering of related insertions [5, 16, 21, 22, 55]. Similarly, missed termination sequences or composite transposition between neighboring Helitrons can lead to the capture of host sequence (gene transduction). In both the bat (Thomas et al. in revision GBE) and maize (for example, ) genomes, thousands of non-autonomous elements carrying gene fragments have been described. Therefore, it is likely that the HINEs might also capture host sequences at some rate, Helentrons carry multiple additional functional genes that were likely captured. Indeed, many functional gene duplications have been described in association with DINE-1 elements [31–34]. It may be that the gene duplications are the result of end bypass or composite rolling-circle transposition.
Captured genes useful to Helentron lifecycle
The Rep/Helicase protein encoded by Helentrons has an endonuclease domain fused to the C-terminus. This endonuclease is most closely related to the proteins encoded by CR1 non-LTR retrotransposons [38, 41] and was either captured when a non-LTR retrotransposed into a Helitron or via a DNA-based transduction event (Thomas et al. in revision GBE). For CR1 retrotransposons target preference is likely determined by the sequence at the 3' end of the element  and is thought to be guided by the endonuclease as has been shown for other non-LTR retrotransposons [45, 57–59]. In target primed reverse transcription, the endonuclease creates the single stranded nick at the target site to initiate the target primed reverse transcription [45, 60]. We hypothesize that the target preference for the poly T track of the Helentrons is guided by the sequence (poly T or A on complementary strand) at the termini of Helentron. As observed with non-LTRs, the endonuclease may be playing a role in target preference and might be generating the single-stranded nick necessary for transposition initiation. We hypothesize that the initial nick in the T sequence flanking the element results in the variability of T nucleotides flanking the elements. Biochemical studies will be necessary to illuminate the mechanism of transposition of these elements.
We find no evidence that either Helentrons or HINEs create target site modification as has sometimes been reported in previous studies [6, 7, 16, 22, 23, 26, 27]. Analysis of the target site is made complicated by target preference for poly T sequences and the presence of an unknown and variable number of T nucleotides flanking individual elements (Figure 2, Additional file 2: Figure S1, Additional file 3: Figure S2). We propose that the variable number of T nucleotides at the termini of the element is the result of the endonuclease indiscriminately cleaving between T dinucleotides in the poly T stretch flanking the element for the initial single stranded cleavage.
The conservation and the probable utilization of the endonuclease for function illustrates that the capture of host genes can lead to innovation in transposition mechanism, which might have downstream consequences to the structure of the elements. Indeed, we identified an intermediate Helentron family that did not encode the endonuclease from Phytophthora (Figure 5). The Rep/Helicase encoded by this family group phylogenetically with the proteins from Helentrons (Additional file 4: Figure S3). However, the element has the structural features of a canonical Helitron and integration occurs between A and T nucleotides.
It is possible that other captured genes (Figure 5, Additional file 9: Table S3) could aid transposition. It has been proposed that the RPA encoded proteins independently captured by Helitrons in plants and Helentrons might bind single stranded DNA during transposition , (for review ). A role of the OTU-like cysteine proteases in transposition is supported by the conservation of all residues required for function and the presence of the genes in Helentron families from diverse species . The protease might be involved in inducing proteolytic events involved in signaling necessary to trigger the modification of chromatin structure [61, 62] or in cleaving the Rep/Helicase/Endonuclease subunits (for review ), which might facilitate transposition. Further experimental studies are necessary to understand mechanisms involved in transposition.
Gene fragment transduction and duplication by Helentrons
We looked for evidence for gene captures using homology based methods in the set of autonomous and non-autonomous elements identified in our study. Besides previously annotated examples, we only identified gene fragments in the Helentrons from the Culex and Phytophthora genomes (Additional file 9: Table S3). We did not detect gene fragments within Helentrons or HINEs as with the frequency observed in maize or bat Helitrons (>10,000 Helitrons containing gene fragments) (Thomas et al. in revision GBE) (for example, . A caveat to this approach is the reliance on significant homology with known proteins. It may be that the frequency of gene captures simply varies with certain genomes, as has been observed with Helitrons.
Previous studies have identified non-random associations of HINE-like elements at or near the break points of several functional gene duplicates in Drosophilids [31–34]. In some of these cases, duplications occurred before as well as after speciation events. For example, for six Kep 1 gene fragment duplications, four duplications occurred before the diversification D. melanogaster species complex (that is, 2-3 to 6-7 million years ago)  while one occurred in D. melanogaster and one occurred in an ancestor of D. sechellia and D. simulans. If the Kep 1 gene fragment is carried by a HINE-like element and duplication occurred as the result of transposition, this pattern would suggest that Helentrons were active throughout the diversification of the D. melanogaster species complex. Other possible gene transduction events include CK2βtes and NACβtes gene duplicates also from the D. melanogaster species complex . The NACβtes copies were amplified before the diversification of D. melangaster species complex and CK2βtes gene duplicates have amplified in the D. sechellia/D. simulans lineage . The hydra gene was uniquely amplified after the insertion of a HINE in the D. melanogaster species complex . A burst of HINE transposition occurred in the ancestor of D. melanogaster species complex and it has been suggested that activity may have continued after diversification . These duplication events do correlate well with the peak of HINE activity (approximately 4.6 million years ago) (average divergence 15.2% ±5.4 SD) . Pairwise divergence (1% to 19%) estimates of the DINE copies in the D. sechellia and D. simulans suggest a longer potential period of activity as compared to D. melanogaster (3-19%) . Helitrons in bats have maintained activity for approximately 36 million years (Thomas et al. in revision GBE). These findings indeed suggest that Helentron activity could play an important role in the amplification and dispersal of genic fragments and the generation of new functional gene duplicates. It maybe that Helentrons like Helitrons are capable of long periods of activity [12, 64].
In this study, we characterized the structural features of Helentrons and identify that they are different from the typical structural features of canonical Helitrons. In addition, we found that Helentrons and DINE-1 like elements share similar structural features, which unequivocally links the DINE-1 like elements as non-autonomous partners of Helentrons. Helentrons and its non-autonomous partners do not induce target site duplications upon transposition, but are flanked by variable number of Ts. We have also identified potential intermediates that have Rep/Helicase protein similar to Helentrons but share the structural characters with Helitrons. Hence our study provides a better understanding of the structure and distribution of Helitron-like elements across taxa. In addition, our studies illustrate that Helentrons are capable of gene transduction as their Helitron relatives.
Identification of HINEs and Helentrons from the mite genome and other selected genomes
Helentrons and HINEs were identified from the genome of the mite, Metaseiulus occidentalis during a de novo analysis of repeats. Repeatscout  was used to generate a consensus of the repeats present in three or more copies in the genome and HINEs were identified during the manual curation of these repeats. The structural characteristics of the HINE elements and Helentrons were compared and analyzed using Blast tools . To identify the distribution pattern, Tblastn searches were carried out using the mite Helentron protein query against wgs, nr, GSS, and HTGS databases. Full-length Helentrons and HINE s were mined from Culex, D. willistoni, D. ananassae, and D. yakuba genomes to verify the relationship between Helentron and HINEs.
Identification of open reading frames, conserved domains, and gene fragments
The Translate (http://web.expasy.org/translate/) and ORF finder (http://www.ncbi.nlm.nih.gov/gorf/gorf.html) tools were utilized to identify Helentron encoded ORFs. The stop codons and frame shift mutations were corrected to obtain an intact the ORF, if necessary. The conserved domain database searches (CDD)  and Blastx were employed to identify putative functional domains of the Helentron encoded proteins and related sequences in other organisms. The low complexity filter was applied during CDD searches to avoid spurious results. Significant hits (e-values <10-03) were further explored. To identify potential protein-coding gene fragments within Helentrons and HINEs, we employed Blastx/CDD based searches and significant (e-values <10-03) hits (other than TEs) were further explored.
Identification of paralogous empty sites
To confirm the boundary of the elements, paralogous sites without the insertion (empty sites) were identified. To identify empty sites, a chimeric query constructed from 50 bp upstream and downstream of the element was utilized for homology-based searches (Blastn) against genomes. Hits with ≥90% identity over 90% of the query are considered as an empty site.
Alignments and phylogenetic analysis
Alignment of the putative proteins encoded by Helentrons, Helitrons, and other bacterial plasmids or viruses that encode similar proteins were constructed employing MUSCLE (http://www.ebi.ac.uk/Tools/msa/muscle/) using default parameters. The alignments were visually refined using Genedoc (v. 2.7), . Phylogenetic analysis of the Rep motifs and helicase domains was conducted using MEGA (v 5.05)  by constructing a NJ tree and the parameters selected were JTT matrix based model, 1,000 bootstrap replicates and pairwise deletion. A maximum likelihood analysis (JTT matrix based model, 1,000 bootstrap replicates) was also performed using MEGA (v 5.05) .
Conserved domain database
Drosophila interspersed element
Helentron associated interspersed element
Jones Taylor Thornton
Long terminal repeat
Miniature inverted terminal repeat elements
Open reading frame
Replication Protein A
Su(var)3-9 and ‘Enhancer of zeste’
Target site duplication.
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This work was supported by start-up funds from the University of Utah to EJP. The funding body has no role in design, in the collection, analysis, and interpretation of data, in the writing of the manuscript; and in the decision to submit the manuscript for publication.
The authors declare that they have no competing interests.
JT conceived, designed, and conducted the study, and drafted the manuscript. KV participated in the study by mining the Phytophthora infestans Helentron and describing the coding capacity. EJP conceived of the study, participated in the design, and drafted the manuscript. All authors read and approved the final manuscript.