Presence Of Intron Research Articles

Evaluation of the incidence of the G143A mutation and cytb intron presence in the cytochrome bc‐1 gene conferring QoI resistance in Botrytis cinerea populations from several hosts

Previous studies have shown that resistance of Botrytis cinerea to QoI fungicides has been attributed to the G143A mutation in the cytochrome b (cytb) gene, while, in a part of the fungal population, an intron has been detected at codon 143 of the gene, preventing QoI resistance. During 2005-2009, 304 grey mould isolates were collected from strawberry, tomato, grape, kiwifruit, cucumber and apple in Greece and screened for resistance to pyraclostrobin and for the presence of the cytb intron, using a novel real-time TaqMan PCR assay developed in the present study. QoI-resistant phenotypes existed only within the population collected from strawberries. All resistant isolates possessed the G143A mutation. Differences were observed in the genotypic structure of cytb. Individuals possessing the intron were found at high incidence in apple fruit and greenhouse-grown tomato and cucumber populations, whereas in the strawberry population the intron frequency was lower. Cultivation of QoI-resistant and QoI-sensitive isolates for ten culture cycles on artificial nutrient medium in the presence or absence of fungicide selection showed that QoI resistance was stable. The results of the study suggest that a high risk for selection of QoI-resistant strains exists in crops heavily treated with QoIs, in spite of the widespread occurrence of the cytb intron in B. cinerea populations. The developed real-time TaqMan PCR constitutes a powerful tool to streamline detection of the mutation by reducing pre- and post-amplification manipulations, and can be used for rapid screening and quantification of QoI resistance.

Exon‐primed, intron‐crossing (EPIC) loci for five nuclear genes in deep‐sea protobranch bivalves: primer design, PCR protocols and locus utility

We describe PCR primers and amplification protocols developed to obtain introns from conserved nuclear genes in deep-sea protobranch bivalves. Because almost no sequence data for protobranchs are publically available, mollusk and other protostome sequences from GenBank were used to design degenerate primers, making these loci potentially useful in other invertebrate taxa. Amplification and sequencing success varied across the test group of 30 species, and we present five loci spanning this range of outcomes. Intron presence in the targeted regions also varied across genes and species, often within single genera; for instance, the calmodulin and β-tubulin loci contained introns with high frequency, whereas the triose phosphate isomerase locus never contained an intron. In introns for which we were able to obtain preliminary estimates of polymorphism levels in single species, polymorphism was greater than traditional mitochondrial loci. These markers will greatly increase the ability to assess population structure in the ecologically important protobranchs, and may prove useful in other taxa as well.

Phylogenetic Distribution of Intron Positions in Alpha-Amylase Genes of Bilateria Suggests Numerous Gains and Losses

Most eukaryotes have at least some genes interrupted by introns. While it is well accepted that introns were already present at moderate density in the last eukaryote common ancestor, the conspicuous diversity of intron density among genomes suggests a complex evolutionary history, with marked differences between phyla. The question of the rates of intron gains and loss in the course of evolution and factors influencing them remains controversial. We have investigated a single gene family, alpha-amylase, in 55 species covering a variety of animal phyla. Comparison of intron positions across phyla suggests a complex history, with a likely ancestral intronless gene undergoing frequent intron loss and gain, leading to extant intron/exon structures that are highly variable, even among species from the same phylum. Because introns are known to play no regulatory role in this gene and there is no alternative splicing, the structural differences may be interpreted more easily: intron positions, sizes, losses or gains may be more likely related to factors linked to splicing mechanisms and requirements, and to recognition of introns and exons, or to more extrinsic factors, such as life cycle and population size. We have shown that intron losses outnumbered gains in recent periods, but that “resets” of intron positions occurred at the origin of several phyla, including vertebrates. Rates of gain and loss appear to be positively correlated. No phase preference was found. We also found evidence for parallel gains and for intron sliding. Presence of introns at given positions was correlated to a strong protosplice consensus sequence AG/G, which was much weaker in the absence of intron. In contrast, recent intron insertions were not associated with a specific sequence. In animal Amy genes, population size and generation time seem to have played only minor roles in shaping gene structures.

Human herpesvirus 6 major immediate early promoter has strong activity in T cells and is useful for heterologous gene expression.

BackgroundHuman herpesvirus-6 (HHV-6) is a beta-herpesvirus. HHV-6 infects and replicates in T cells. The HHV-6-encoded major immediate early gene (MIE) is expressed at the immediate-early infection phase. Human cytomegalovirus major immediate early promoter (CMV MIEp) is commercially available for the expression of various heterologous genes. Here we identified the HHV-6 MIE promoter (MIEp) and compared its activity with that of CMV MIEp in various cell lines.MethodsThe HHV-6 MIEp and some HHV-6 MIEp variants were amplified by PCR from HHV-6B strain HST. These fragments and CMV MIEp were subcloned into the pGL-3 luciferase reporter plasmid and subjected to luciferase reporter assay. In addition, to investigate whether the HHV-6 MIEp could be used as the promoter for expression of foreign genes in a recombinant varicella-zoster virus, we inserted HHV-6 MIEp-DsRed expression casette into the varicella-zoster virus genome.ResultsHHV-6 MIEp showed strong activity in T cells compared with CMV MIEp, and the presence of intron 1 of the MIE gene increased its activity. The NF-κB-binding site, which lies within the R3 repeat, was critical for this activity. Moreover, the HHV-6 MIEp drove heterologous gene expression in recombinant varicella-zoster virus-infected cells.ConclusionsThese data suggest that HHV-6 MIEp functions more strongly than CMV MIEp in various T-cell lines.

Introns reduce transitivity proportionally to their length, suggesting that silencing spreads along the pre-mRNA

Endogenes rarely support transitive silencing, whereas most transgenes generally allow the spread of silencing to occur along the primary target. To determine whether the presence of introns might explain the difference, we investigated the influence of introns in the primary target on 3'–5' silencing transitivity. When present in a transgene, an intron-containing endogene fragment does not prohibit the spread of silencing across this fragment, indicating that introns do not preclude silencing transitivity along endogenes. Also, a multiple intron-containing genomic gene fragment that had previously been shown not to support transitivity in an endogenous context could support transitivity when present in a transgene. Nevertheless, genomic intron-containing fragments delayed the onset and diminished the efficiency of transitive silencing of a secondary target compared with the corresponding cDNA fragments. Remarkably, transitivity was impaired proportionally with the length of the pre-mRNA, and not of the mRNA. The latter result suggests that the RNA dependent RNA polymerase-based spreading of silencing progresses along the non-spliced rather than the fully processed mature mRNA.

Development of unigene-derived SSR markers in cowpea (Vigna unguiculata) and their transferability to otherVignaspecies

Unigene sequences available in public databases provide a cost-effective and valuable source for the development of molecular markers. In this study, the identification and development of unigene-based SSR markers in cowpea (Vigna unguiculata (L.) Walp.) is presented. A total of 1071 SSRs were identified in 15 740 cowpea unigene sequences downloaded from the National Center for Biotechnology Information. The most frequent SSR motifs present in the unigenes were trinucleotides (59.7%), followed by dinucleotides (34.8%), pentanucleotides (4%), and tetranucleotides (1.5%). The copy number varied from 6 to 33 for dinucleotide, 5 to 29 for trinucleotide, 5 to 7 for tetranucleotide, and 4 to 6 for pentanucleotide repeats. Primer pairs were successfully designed for 803 SSR motifs and 102 SSR markers were finally characterized and validated. Putative function was assigned to 64.7% of the unigene SSR markers based on significant homology to reported proteins. About 31.7% of the SSRs were present in coding sequences and 68.3% in untranslated regions of the genes. About 87% of the SSRs located in the coding sequences were trinucleotide repeats. Allelic variation at 32 SSR loci produced 98 alleles in 20 cowpea genotypes. The polymorphic information content for the SSR markers varied from 0.10 to 0.83 with an average of 0.53. These unigene SSR markers showed a high rate of transferability (88%) across other Vigna species, thereby expanding their utility. Alignment of unigene sequences with soybean genomic sequences revealed the presence of introns in amplified products of some of the SSR markers. This study presents the distribution of SSRs in the expressed portion of the cowpea genome and is the first report of the development of functional unigene-based SSR markers in cowpea. These SSR markers would play an important role in molecular mapping, comparative genomics, and marker-assisted selection strategies in cowpea and other Vigna species.

A Possible Role for Short Introns in the Acquisition of Stroma-Targeting Peptides in the Flagellate Euglena gracilis

The chloroplasts of Euglena gracilis bounded by three membranes arose via secondary endosymbiosis of a green alga in a heterotrophic euglenozoan host. Many genes were transferred from symbiont to the host nucleus. A subset of Euglena nuclear genes of predominately symbiont, but also host, or other origin have obtained complex presequences required for chloroplast targeting. This study has revealed the presence of short introns (41–93 bp) either in the second half of presequence-encoding regions or shortly downstream of them in nine nucleus-encoded E. gracilis genes for chloroplast proteins (Eno29, GapA, PetA, PetF, PetJ, PsaF, PsbM, PsbO, and PsbW). In addition, the E. gracilis Pbgd gene contains two introns in the second half of presequence-encoding region and one at the border of presequence-mature peptide-encoding region. Ten of 12 introns present within presequence-encoding regions or shortly downstream of them identified in this study have typical eukaryotic GT/AG borders, are T-rich, 45–50 bp long, and pairwise sequence identities range from 27 to 61%. Thus single recombination events might have been mediated via these cis-spliced introns. A double crossing over between these cis-spliced introns and trans-spliced introns present in 5′-UTRs of Euglena nuclear genes is also likely to have occurred. Thus introns and exon-shuffling could have had an important role in the acquisition of chloroplast targeting signals in E. gracilis. The results are consistent with a late origin of photosynthetic euglenids.

A constructive step towards selecting a DNA barcode for fungi

A constructive step towards selecting a DNA barcode for fungi

DMR1 (CCM1/YGR150C) of Saccharomyces cerevisiae Encodes an RNA-Binding Protein from the Pentatricopeptide Repeat Family Required for the Maintenance of the Mitochondrial 15S Ribosomal RNA

Pentatricopeptide repeat (PPR) proteins form the largest known RNA-binding protein family and are found in all eukaryotes, being particularly abundant in higher plants. PPR proteins localize mostly in mitochondria and chloroplasts, where they modulate organellar genome expression on the post-transcriptional level. The Saccharomyces cerevisiae DMR1 (CCM1, YGR150C) encodes a PPR protein that localizes to mitochondria. Deletion of DMR1 results in a complete and irreversible loss of respiratory capacity and loss of wild-type mtDNA by conversion to rho(-)/rho(0) petites, regardless of the presence of introns in mtDNA. The phenotype of the dmr1Delta mitochondria is characterized by fragmentation of the small subunit mitochondrial rRNA (15S rRNA), that can be reversed by wild-type Dmr1p. Other mitochondrial transcripts, including the large subunit mitochondrial rRNA (21S rRNA), are not affected by the lack of Dmr1p. The purified Dmr1 protein specifically binds to different regions of 15S rRNA in vitro, consistent with the deletion phenotype. Dmr1p is therefore the first yeast PPR protein, which has an rRNA target and is probably involved in the biogenesis of mitochondrial ribosomes and translation.

A new Ophiostoma species from loblolly pine roots in the southeastern United States

During the course of a survey of fungi in loblolly pine (Pinus taeda) roots in Georgia, USA, a species of Ophiostoma morphologically similar to O. pluriannulatum, was isolated. Morphological characteristics and DNA sequence comparisons were used to identify the fungus. The isolates produced perithecia with unusually long necks similar to those of O. pluriannulatum but they had few or no annuli. DNA sequences for the ribosomal internal transcribed spacer regions 1 and 2 were identical to those of O. pluriannulatum. Sequence data of the β-tubulin gene region revealed the absence of intron 4 and presence of intron 5, distinguishing the isolates from O. pluriannulatum, which has intron 4 but not intron 5. Phylogenetic analyses of the β-tubulin sequences showed that the isolates from loblolly pine roots grouped together in a lineage distinct from O. multiannulatum and O. subannulatum, both of which lack intron 4 and have intron 5. The fungus is consequently described as O. sparsiannulatum sp. nov., a novel taxon in the O. pluriannulatum complex.

Genome-wide functional analysis of human 5' untranslated region introns

Genes with short 5'UTR introns have higher expression than genes with no or long 5'UTR introns. Complex evolutionary forces act on these introns.

The upstream open reading frame of the Arabidopsis AtMHX gene has a strong impact on transcript accumulation through the nonsense‐mediated mRNA decay pathway

Approximately 20% of plant genes possess upstream open-reading frames (uORFs). The effect of uORFs on gene expression has mainly been studied at the translational level. Very little is known about the impact of plant uORFs on transcript content through the nonsense-mediated mRNA decay (NMD) pathway, which degrades transcripts bearing premature termination codons (PTCs). Here we examine the impact of the uORF of the Arabidopsis AtMHX gene on transcript accumulation. The suggestion that this uORF exposes transcripts containing it to NMD is supported by (i) the increase in transcript levels upon eliminating the uORF from constructs containing it, (ii) experiments with a modified uORF-peptide, which excluded peptide-specific degradation mechanisms, (iii) the increase in levels of the native AtMHX transcript upon treatment with cycloheximide, which inhibits translation and blocks NMD, and (iv) the sensitivity of transcripts containing the uORF of AtMHX to the presence of introns. We also showed that introns can increase NMD efficiency not only in transcripts having relatively short 3' untranslated regions (UTRs), but also in uORF-containing transcripts. AtMHX transcript levels were almost unaltered in mutants of the NMD factors UPF3 and UPF1. Possible reasons, including the existence of a NMD-compensatory mechanism, are discussed. Interestingly, the levels of UPF3 transcript were higher in upf1 mutants, suggesting a compensatory mechanism that links weak function of the NMD machinery to increased expression of UPF3. Our findings highlight that uORFs, which are abundant in plants, can not only inhibit translation but also strongly affect transcript accumulation.

Chronic Eosinophilic Leukaemia (CEL) with FIP1L1-PDGFRalpha Rearrangement (F/P): The Response to Imatinib (IM) Is Durable. A Report of 33 Patients with A Follow –up of 30 to 92 Months.

Abstract 3894Poster Board III-830 BackgroundAn interstitial deletion in the long arm of chromosome 4 leads to the formation of the fusion gene FIP1L1-PDGFRalpha (F/P) coding for a constitutively activated form of PDGFRalpha. The fusion gene represents a marker of CEL, and it predicts a dramatic response to imatinib mesylate (IM). Different F/P transcripts have been described, but the correlation with kinetic of molecular response to IM and with the clinical presentation is unknown. Remission duration is also still undefined. AimThe aim of this study was to evaluate the duration of IM response in F/P positive (F/P+) CEL patients and the correlation between their clinical behaviour and molecular characteristics of the transcripts. Design and methodsA prospective phase 2 multicentre study of the use of IM 400 mg/daily in patients with hypereosinophilic syndrome, independently of F/P status was established in 2001. Hypereosinophilic syndrome was defined according to Chusid criteria (Blood, 1994; 83: 2759-2779). The presence of F/P transcript was investigated on bone marrow cells using a nested reverse transcriptase polymerase chain reaction (RT-PCR), as reported by Cools J (N Engl J Med. 2003; 348:1199-200). The samples were also purified and sequenced using the Big Dye Terminator Cycle Sequencing Kit (Applied Biosystems). Patients at diagnosis were systematically screened for organ damage with instrumental evaluation (chest radiography, echocardiogram, abdomen ultrasonography) and for the presence of symptoms. 72 patients were treated with IM 100 to 400 mg daily; the 33 F/P+ were monitored every three months for two years then every six months using nested RT-PCR. We will present the result of the 33 F/P+ patients. ResultsMedian follow-up was of 51 months (range 30-92 months). There were 32 males and one female patients. Organ involvement was recorded in 42% of patients, with notably no skin involvement, splenomegaly reported in 7, cardiac involvement in 5, and in 2 cases the peculiar site of localization of soft tissue. After imatinib therapy all patients achieved a complete hematologic response (CHR) in less than one month, and PCR negativity in a median time of 3 months (range 1-9). They became negative for organ localizations and free of symptoms. No patient experienced cardiac failure. All patients who continue imatinib therapy remained in CHR and RT-PCR negative, with a dose of 100 to 400 mg daily. From September 2007 all patients except one (late responder) were treated with 100 mg daily. In six patients IM treatment was discontinued for variable period for different reasons, and in 5 cases the fusion transcript became rapidly detectable; CHR was maintained, other than in one case and the transcript was again undetectable upon treatment resumption, other than in one case. Twenty-eight samples were evaluable for sequencing. Molecular analysis demonstrate an extreme variability of FIP1L1-PDGFRA junction sequences, with FIP1L1 breackpoints scattered between exon 9 to 18, with several splicing variants, whereas all breackpoints in PDGFRA are located within exon 12. Fusion gene sequencing demonstrate an extreme variability, with lack of whole exons of FIP1L1, deletion of exons, with the presence of introns in a minority of cases. Region of FIP1L1 varies in length from 109 to more than 500 nucleotides. Transcript of the only female patient is the same of one of the males. No evidence of correlation was noted with kinetic of molecular response or with the presence at diagnosis of peculiar organ involvement. More complexity of transcript is noted in patients with longer history of disease prior to imatinib therapy. Interpretation and conclusionwith this large series of patients we can confirm that the response to imatinib therapy is durable, is not influenced by the biologic features, but depends on continuous therapy. More complexity and variability in FIP1L1-PDGFRalpha transcripts does not lead to differences in terms of response to the therapy or phenotype of disease at diagnosis. Disclosures:Pane:Novartis: Research Funding; Ministero dell'Università/PRIN: Research Funding; Regione Campania: Research Funding; Ministero della Salute/Progetto integrato Oncologia: Research Funding. Saglio:Novartis: Honoraria; Celgene: Honoraria. Baccarani:Novartis Pharma: Consultancy, Honoraria, Research Funding, Speakers Bureau; Bristol-Mayer Squibb: Consultancy, Honoraria, Research Funding, Speakers Bureau.

Evolutionary genetic insights into Plasmodium falciparum functional genes

Complex and rapidly evolving behavior of the human malaria parasite Plasmodium falciparum have always been mysterious to the evolutionary biologists, as the parasite is the most virulent and now becoming the most prevalent malaria parasite species across the globe. With the availability of complete genome sequence of P. falciparum, better understanding of the genome design and evolution could be possible. We herein utilized the available information of all known functional genes from whole genome of P. falciparum and investigate the differential mode of gene evolution. The study comparing P. falciparum functional genes with Plasmodium vivax revealed about 82% of genes to be conserved in the later species and the rest, 18% to be totally unique to P. falciparum. Genetic architectural pattern of functional genes shows absence of introns in about a half of the conserved genes, whereas almost all unique genes have introns. Similarly, distribution of intron number and length were also observed to be different for conserved and unique genes of P. falciparum. Statistically significant positive correlations between total intron length and gene lengths were detected in 11 chromosomes for unique genes, whereas only in three chromosomes for conserved genes. Preference of intron presence in some P. falciparum genes were also detected which provide functional relevance of introns. The study provides, for the first time, a detail evolutionary analysis of functional genes of a devastating malaria parasite. The marked differences in organization of introns between the unique and conserved genes in P. falciparum, and the contribution of introns to genome complexity are some of the hallmarks of the study.

Endogenous Mechanisms for the Origins of Spliceosomal Introns

Over 30 years since their discovery, the origin of spliceosomal introns remains uncertain. One nearly universally accepted hypothesis maintains that spliceosomal introns originated from self-splicing group-II introns that invaded the uninterrupted genes of the last eukaryotic common ancestor (LECA) and proliferated by "insertion" events. Although this is a possible explanation for the original presence of introns and splicing machinery, the emphasis on a high number of insertion events in the genome of the LECA neglects a considerable body of empirical evidence showing that spliceosomal introns can simply arise from coding or, more generally, nonintronic sequences within genes. After presenting a concise overview of some of the most common hypotheses and mechanisms for intron origin, we propose two further hypotheses that are broadly based on central cellular processes: 1) internal gene duplication and 2) the response to aberrant and fortuitously spliced transcripts. These two nonmutually exclusive hypotheses provide a powerful way to explain the establishment of spliceosomal introns in eukaryotes without invoking an exogenous source.

Towards barcode markers in Fungi: an intron map of Ascomycota mitochondria

BackgroundA standardized and cost-effective molecular identification system is now an urgent need for Fungi owing to their wide involvement in human life quality. In particular the potential use of mitochondrial DNA species markers has been taken in account. Unfortunately, a serious difficulty in the PCR and bioinformatic surveys is due to the presence of mobile introns in almost all the fungal mitochondrial genes. The aim of this work is to verify the incidence of this phenomenon in Ascomycota, testing, at the same time, a new bioinformatic tool for extracting and managing sequence databases annotations, in order to identify the mitochondrial gene regions where introns are missing so as to propose them as species markers.MethodsThe general trend towards a large occurrence of introns in the mitochondrial genome of Fungi has been confirmed in Ascomycota by an extensive bioinformatic analysis, performed on all the entries concerning 11 mitochondrial protein coding genes and 2 mitochondrial rRNA (ribosomal RNA) specifying genes, belonging to this phylum, available in public nucleotide sequence databases. A new query approach has been developed to retrieve effectively introns information included in these entries.ResultsAfter comparing the new query-based approach with a blast-based procedure, with the aim of designing a faithful Ascomycota mitochondrial intron map, the first method appeared clearly the most accurate. Within this map, despite the large pervasiveness of introns, it is possible to distinguish specific regions comprised in several genes, including the full NADH dehydrogenase subunit 6 (ND6) gene, which could be considered as barcode candidates for Ascomycota due to their paucity of introns and to their length, above 400 bp, comparable to the lower end size of the length range of barcodes successfully used in animals.ConclusionThe development of the new query system described here would answer the pressing requirement to improve drastically the bioinformatics support to the DNA Barcode Initiative. The large scale investigation of Ascomycota mitochondrial introns performed through this tool, allowing to exclude the introns-rich sequences from the barcode candidates exploration, could be the first step towards a mitochondrial barcoding strategy for these organisms, similar to the standard approach employed in metazoans.

FGLamide Allatostatin genes in Arthropoda: Introns early or late?

FGLamide allatostatins are invertebrate neuropeptides which inhibit juvenile hormone biosynthesis in Dictyoptera and related orders and also show myomodulatory activity. The FGLamide allatostatin (AST) gene structure in Dictyoptera is intronless within the ORF, whereas in 9 species of Diptera, the FGLamide AST ORF has one intron. To investigate the evolutionary history of AST intron structure, (intron early versus intron late hypothesis), all available Arthropoda FGLamide AST gene sequences were examined from genome databases with reference to intron presence and position/phase. Three types of FGLamide AST ORF organization were found: intronless in I. scapularis and P. humanus corporis; one intron in D. pulex, A. pisum, A. mellifera and five Drosophila sp.; two introns in N. vitripennis, B. mori strains, A. aegypti, A. gambiae and C. quinquefasciatus. The literature suggests that for the majority of genes examined, most introns exist between codons (phase 0) which may reflect an ancient function of introns to separate protein modules. 60% of the FGLamide AST ORFs introns were between the first and second base within a codon (phase 1), 28% were between the second and third nucleotides within a codon (phase two) and 12% were phase 0. As would be required for correct intron splicing consensus sequence, 84% of introns were in codons starting with guanine. The positioning of introns was a maximum of 9 codons from a dibasic cleavage site. Our results suggest that the introns in the analyzed species support the intron late model.

Gene structures and processing of Arabidopsis thaliana HYL1-dependent pri-miRNAs

Arabidopsis thaliana HYL1 is a nuclear double-stranded RNA-binding protein involved in the maturation of pri-miRNAs. A quantitative real-time PCR platform for parallel quantification of 176 pri-miRNAs was used to reveal strong accumulation of 57 miRNA precursors in the hyl1 mutant that completely lacks HYL1 protein. This approach enabled us for the first time to pinpoint particular members of MIRNA family genes that require HYL1 activity for efficient maturation of their precursors. Moreover, the accumulation of miRNA precursors in the hyl1 mutant gave us the opportunity to carry out 3′ and 5′ RACE experiments which revealed that some of these precursors are of unexpected length. The alignment of HYL1-dependent miRNA precursors to A. thaliana genomic sequences indicated the presence of introns in 12 out of 20 genes studied. Some of the characterized intron-containing pri-miRNAs undergo alternative splicing such as exon skipping or usage of alternative 5′ splice sites suggesting that this process plays a role in the regulation of miRNA biogenesis. In the hyl1 mutant intron-containing pri-miRNAs accumulate alongside spliced pri-miRNAs suggesting the recruitment of HYL1 into the miRNA precursor maturation pathway before their splicing occurs.

RasiRNA pathway controls antisense expression of Drosophila telomeric retrotransposons in the nucleus

Telomeres in Drosophila are maintained by the specialized telomeric retrotransposons HeT-A, TART and TAHRE. Sense transcripts of telomeric retroelements were shown to be the targets of a specialized RNA-interference mechanism, a repeat-associated short interfering (rasi)RNA-mediated system. Antisense rasiRNAs play a key role in this mechanism, highlighting the importance of antisense expression in retrotransposon silencing. Previously, bidirectional transcription was reported for the telomeric element TART. Here, we show that HeT-A is also bidirectionally transcribed, and HeT-A antisense transcription in ovaries is regulated by a promoter localized within its 3′ untranslated region. A remarkable feature of noncoding HeT-A antisense transcripts is the presence of multiple introns. We demonstrate that sense and antisense HeT-A-specific rasiRNAs are present in the same tissue, indicating that transcripts of both directions may be considered as natural targets of the rasiRNA pathway. We found that the expression of antisense transcripts of telomeric elements is regulated by the RNA silencing machinery, suggesting rasiRNA-mediated interplay between sense and antisense transcripts in the cell. Finally, this regulation occurs in the nucleus since disruption of the rasiRNA pathway leads to an accumulation of TART and HeT-A transcripts in germ cell nuclei.

Transfer RNA genes in pieces are an ancestral character

Randau & Soll (2008) suggest that the anticodon loop region of transfer RNA (tRNA) genes is the target site of a huge variety of mobile genetic elements and that this resulted in the evolution of tRNA genes in pieces. They therefore suggest that the presence of introns in the anticodon loop might not be a plesiomorphic trait that is important in tRNA origin (Di Giulio, 2006), but an acquired trait, as it endowed tRNA genes with a precious protection mechanism against the integration of viruses and other autonomous genetic elements, in that the intron removed the integration site of these mobile genetic elements. Similarly, Randau & Soll maintain that split and permuted tRNA genes might also have been able to prevent the integration of mobile genetic elements, as their attachment sites were evidently disrupted and hence restricted their propagation within the genome, thus conferring a selective advantage. This evolution should not have been possible …