Reverse transcriptase template switching and false alternative transcripts

RNA sequencing: advances, challenges and opportunities

In the few years since its initial application, massively parallel cDNA sequencing, or RNA-seq, has allowed many advances in the characterization and quantification of transcriptomes. Recently, several developments in RNA-seq methods have provided an even more complete characterization of RNA transcripts. These developments include improvements in transcription start site mapping, strand-specific measurements, gene fusion detection, small RNA characterization and detection of alternative splicing events. Ongoing developments promise further advances in the application of RNA-seq, particularly direct RNA sequencing and approaches that allow RNA quantification from very small amounts of cellular materials.

Expression of ribosomal protein L22e family members in Drosophila melanogaster: rpL22-like is differentially expressed and alternatively spliced

Several ribosomal protein families contain paralogues whose roles may be equivalent or specialized to include extra-ribosomal functions. RpL22e family members rpL22 and rpL22-like are differentially expressed in Drosophila melanogaster: rpL22-like mRNA is gonad specific whereas rpL22 is expressed ubiquitously, suggesting distinctive paralogue functions. To determine if RpL22-like has a divergent role in gonads, rpL22-like expression was analysed by qRT-PCR and western blots, respectively, showing enrichment of rpL22-like mRNA and a 34 kDa (predicted) protein in testis, but not in ovary. Immunohistochemistry of the reproductive tract corroborated testis-specific expression. RpL22-like detection in 80S/polysome fractions from males establishes a role for this tissue-specific paralogue as a ribosomal component. Unpredictably, expression profiles revealed a low abundant, alternative mRNA variant (designated 'rpL22-like short') that would encode a novel protein lacking the C-terminal ribosomal protein signature but retaining part of the N-terminal domain. This variant results from splicing of a retained intron (defined by non-canonical splice sites) within rpL22-like mRNA. Polysome association and detection of a low abundant 13.5 kDa (predicted) protein in testis extracts suggests variant mRNA translation. Collectively, our data show that alternative splicing of rpL22-like generates structurally distinct protein products: ribosomal component RpL22-like and a novel protein with a role distinct from RpL22-like.

Recombination of 5' subgenomic RNA3a with genomic RNA3 of Brome mosaic bromovirus in vitro and in vivo

RNA-RNA recombination salvages viral RNAs and contributes to their genomic variability. A recombinationally-active subgenomic promoter (sgp) has been mapped in Brome mosaic bromovirus (BMV) RNA3 (Wierzchoslawski et al., 2004. J. Virol.78, 8552-8864) and mRNA-like 5' sgRNA3a was characterized (Wierzchoslawski et al., 2006. J. Virol. 80, 12357-12366). In this paper we describe sgRNA3a-mediated recombination in both in vitro and in vivo experiments. BMV replicase-directed co-copying of (-) RNA3 with wt sgRNA3a generated RNA3 recombinants in vitro, but it failed to when 3'-truncated sgRNA3a was substituted, demonstrating a role for the 3' polyA tail. Barley protoplast co-transfections revealed that (i) wt sgRNA3a recombines at the 3' and the internal sites; (ii) 3'-truncated sgRNA3as recombine more upstream; and (iii) 5'-truncated sgRNA3 recombine at a low rate. In planta co-inoculations confirmed the RNA3-sgRNA3a crossovers. In summary, the non-replicating sgRNA3a recombines with replicating RNA3, most likely via primer extension and/or internal template switching.

A critical analysis of Atoh7 (Math5) mRNA splicing in the developing mouse retina

The Math5 (Atoh7) gene is transiently expressed during retinogenesis by progenitors exiting mitosis, and is essential for ganglion cell (RGC) development. Math5 contains a single exon, and its 1.7 kb mRNA encodes a 149-aa polypeptide. Mouse Math5 mutants have essentially no RGCs or optic nerves. Given the importance of this gene in retinal development, we thoroughly investigated the possibility of Math5 mRNA splicing by Northern blot, 3'RACE, RNase protection assays, and RT-PCR, using RNAs extracted from embryonic eyes and adult cerebellum, or transcribed in vitro from cDNA clones. Because Math5 mRNA contains an elevated G+C content, we used graded concentrations of betaine, an isostabilizing agent that disrupts secondary structure. Although approximately 10% of cerebellar Math5 RNAs are spliced, truncating the polypeptide, our results show few, if any, spliced Math5 transcripts exist in the developing retina (<1%). Rare deleted cDNAs do arise via RT-mediated RNA template switching in vitro, and are selectively amplified during PCR. These data differ starkly from a recent study (Kanadia and Cepko 2010), which concluded that the vast majority of Math5 and other bHLH transcripts are spliced to generate noncoding RNAs. Our findings clarify the architecture of the Math5 gene and its mechanism of action. These results have implications for all members of the bHLH gene family, for any gene that is alternatively spliced, and for the interpretation of all RT-PCR experiments.

Apparent non-canonical trans-splicing is generated by reverse transcriptase in vitro

BACKGROUND

Trans-splicing, the in vivo joining of two independently transcribed RNA molecules, is well characterized in lower eukaryotes, but was long thought absent from metazoans. However, recent bioinformatic analyses of EST sequences suggested widespread trans-splicing in mammals. These apparently spliced transcripts generally lacked canonical splice sites, leading us to question their authenticity. Particularly, the native ability of reverse transcriptase enzymes to template switch during transcription could produce apparently trans-spliced sequences.

PRINCIPAL FINDINGS

Here we report an in vitro system for the analysis of template switching in reverse transcription. Using highly purified RNA substrates, we show the reproducible occurrence of apparent trans-splicing between two RNA molecules. Other reported non-canonical splicing events such as exon shuffling and sense-antisense fusions were also readily detected. The latter caused the production of apparent antisense non-coding RNAs, which are also reported to be abundant in humans.

CONCLUSIONS

We propose that most reported examples of non-canonical splicing in metazoans arise through template switching by reverse transcriptase during cDNA preparation. We further show that the products of template switching can vary between reverse transcriptases, providing a simple diagnostic for identifying many of these experimental artifacts.

Global analysis of trans-splicing in Drosophila

Precursor mRNA (pre-mRNA) splicing can join exons contained on either a single pre-mRNA (cis) or on separate pre-mRNAs (trans). It is exceedingly rare to have trans-splicing between protein-coding exons and has been demonstrated for only two Drosophila genes: mod(mdg4) and lola. It has also been suggested that trans-splicing is a mechanism for the generation of chimeric RNA products containing sequence from multiple distant genomic sites. Because most high-throughput approaches cannot distinguish cis- and trans-splicing events, the extent to which trans-splicing occurs between protein-coding exons in any organism is unknown. Here, we used paired-end deep sequencing of mRNA to identify genes that undergo trans-splicing in Drosophila interspecies hybrids. We did not observe credible evidence for the existence of chimeric RNAs generated by trans-splicing of RNAs transcribed from distant genomic loci. Rather, our data suggest that experimental artifacts are the source of most, if not all, apparent chimeric RNA products. We did, however, identify 80 genes that appear to undergo trans-splicing between homologous alleles and can be classified into three categories based on their organization: (i) genes with multiple 3' terminal exons, (ii) genes with multiple first exons, and (iii) genes with very large introns, often containing other genes. Our results suggest that trans-splicing between homologous alleles occurs more commonly in Drosophila than previously believed and may facilitate expression of architecturally complex genes.

A variant of the KLK4 gene is expressed as a cis sense-antisense chimeric transcript in prostate cancer cells

In humans, more than 30,000 chimeric transcripts originating from 23,686 genes have been identified. The mechanisms and association of chimeric transcripts arising from chromosomal rearrangements with cancer are well established, but much remains unknown regarding the biogenesis and importance of other chimeric transcripts that arise from nongenomic alterations. Recently, a SLC45A3-ELK4 chimera has been shown to be androgen-regulated, and is overexpressed in metastatic or high-grade prostate tumors relative to local prostate cancers. Here, we characterize the expression of a KLK4 cis sense-antisense chimeric transcript, and show other examples in prostate cancer. Using non-protein-coding microarray analyses, we initially identified an androgen-regulated antisense transcript within the 3' untranslated region of the KLK4 gene in LNCaP cells. The KLK4 cis-NAT was validated by strand-specific linker-mediated RT-PCR and Northern blotting. Characterization of the KLK4 cis-NAT by 5' and 3' rapid amplification of cDNA ends (RACE) revealed that this transcript forms multiple fusions with the KLK4 sense transcript. Lack of KLK4 antisense promoter activity using reporter assays suggests that these transcripts are unlikely to arise from a trans-splicing mechanism. 5' RACE and analyses of deep sequencing data from LNCaP cells treated +/-androgens revealed six high-confidence sense-antisense chimeras of which three were supported by the cDNA databases. In this study, we have shown complex gene expression at the KLK4 locus that might be a hallmark of cis sense-antisense chimeric transcription.

Structure and function of splice variants of the cardiac voltage-gated sodium channel Na(v)1.5

Voltage-gated sodium channels mediate the rapid upstroke of the action potential in excitable tissues. The tetrodotoxin (TTX) resistant isoform Na(v)1.5, encoded by the SCN5A gene, is the predominant isoform in the heart. This channel plays a key role for excitability of atrial and ventricular cardiomyocytes and for rapid impulse propagation through the specific conduction system. During recent years, strong evidence has been accumulated in support of the expression of several Na(v)1.5 splice variants in the heart, and in various other tissues and cell lines including brain, dorsal root ganglia, breast cancer cells and neuronal stem cell lines. This review summarizes our knowledge on the structure and putative function of nine Na(v)1.5 splice variants detected so far. Attention will be paid to the distinct biophysical properties of the four functional splice variants, to the pronounced tissue- and species-specific expression, and to the developmental regulation of Na(v)1.5 splicing. The implications of alternative splicing for SCN5A channelopathies, and for a better understanding of genotype-phenotype correlations, are discussed.

Expression profiling of Drosophila mitochondrial genes via deep mRNA sequencing

Mitochondria play an essential role in several cellular processes. Nevertheless, very little is known about patterns of gene expression of genes encoded by the mitochondrial DNA (mtDNA). In this study, we used next-generation sequencing (NGS) for transcription profiling of genes encoded in the mitochondrial genome of Drosophila melanogaster and D. pseudoobscura. The analysis of males and females in both species indicated that the expression pattern was conserved between the two species, but differed significantly between both sexes. Interestingly, mRNA levels were not only different among genes encoded by separate transcription units, but also showed significant differences among genes located in the same transcription unit. Hence, mRNA abundance of genes encoded by mtDNA seems to be heavily modulated by post-transcriptional regulation. Finally, we also identified several transcripts with a noncanonical structure, suggesting that processing of mitochondrial transcripts may be more complex than previously assumed.

Ubiquitous internal gene duplication and intron creation in eukaryotes

Duplication of genomic segments provides a primary resource for the origin of evolutionary novelties. However, most previous studies have focused on duplications of complete protein-coding genes, whereas little is known about the significance of duplication segments that are entirely internal to genes. Our examination of six fully sequenced genomes reveals that internal duplications of gene segments occur at a high frequency (0.001-0.013 duplications/gene per million years), similar to that of complete gene duplications, such that 8-17% of the genes in a genome carry duplicated intronic and/or exonic regions. At least 7-30% of such genes have acquired novel introns, either because a prior intron in the same gene has been duplicated, or more commonly, because a spatial change has activated a latent splice site. These results strongly suggest a major evolutionary role for internal gene duplications in the origin of genomic novelties, particularly as a mechanism for intron gain.

Direct RNA sequencing

Our understanding of human biology and disease is ultimately dependent on a complete understanding of the genome and its functions. The recent application of microarray and sequencing technologies to transcriptomics has changed the simplistic view of transcriptomes to a more complicated view of genome-wide transcription where a large fraction of transcripts emanates from unannotated parts of genomes, and underlined our limited knowledge of the dynamic state of transcription. Most of this broad body of knowledge was obtained indirectly because current transcriptome analysis methods typically require RNA to be converted to complementary DNA (cDNA) before measurements, even though the cDNA synthesis step introduces multiple biases and artefacts that interfere with both the proper characterization and quantification of transcripts. Furthermore, cDNA synthesis is not particularly suitable for the analysis of short, degraded and/or small quantity RNA samples. Here we report direct single molecule RNA sequencing without prior conversion of RNA to cDNA. We applied this technology to sequence femtomole quantities of poly(A)(+) Saccharomyces cerevisiae RNA using a surface coated with poly(dT) oligonucleotides to capture the RNAs at their natural poly(A) tails and initiate sequencing by synthesis. We observed transcript 3' end heterogeneity and polyadenylated small nucleolar RNAs. This study provides a path to high-throughput and low-cost direct RNA sequencing and achieving the ultimate goal of a comprehensive and bias-free understanding of transcriptomes.

MCL-1ES, a novel variant of MCL-1, associates with MCL-1L and induces mitochondrial cell death

Myeloid cell leukemia-1 (MCL-1L) is a pro-survival member of the BCL-2 family that promotes cell survival. In this study, we identify a new splicing variant of human MCL-1 that encodes MCL-1ES (extra short). Sequence analysis indicates that this variant results from splicing within the first coding exon of MCL-1 at a non-canonical GC-AG donor-acceptor pair. The deduced sequence of MCL-1ES encodes a protein of 197 amino acids, and the PEST (proline, glutamic acid, serine, and threonine) motifs present in MCL-1L are absent. MCL-1ES interacts with MCL-1L and induces mitochondrial cell death, suggesting that alternative splicing of MCL-1 may control the fate of cells.

Mystery of intron gain: new data and new models

Despite their ubiquity, the mechanisms and evolutionary forces responsible for the origins of spliceosomal introns remain mysterious. Recent molecular evidence supports the idea that intronic RNAs can reverse splice into RNA transcripts, a crucial step for an influential model of intron gain. However, a paradox attends this model because the rate of intron gain is expected to be orders of magnitude lower than the rate of intron loss in general, in contrast to findings from several lineages. We suggest two possible resolutions to this paradox, based on steric considerations and on the possibility of co-option by specific introns of retroelement transposition pathways, respectively. In addition, we introduce two potential mechanisms for intron creation, based on hybrid RNA-DNA reverse splicing and on template switching errors by reverse transcriptase.

Evolutionary convergence on highly-conserved 3' intron structures in intron-poor eukaryotes and insights into the ancestral eukaryotic genome

The presence of spliceosomal introns in eukaryotes raises a range of questions about genomic evolution. Along with the fundamental mysteries of introns' initial proliferation and persistence, the evolutionary forces acting on intron sequences remain largely mysterious. Intron number varies across species from a few introns per genome to several introns per gene, and the elements of intron sequences directly implicated in splicing vary from degenerate to strict consensus motifs. We report a 50-species comparative genomic study of intron sequences across most eukaryotic groups. We find two broad and striking patterns. First, we find that some highly intron-poor lineages have undergone evolutionary convergence to strong 3' consensus intron structures. This finding holds for both branch point sequence and distance between the branch point and the 3' splice site. Interestingly, this difference appears to exist within the genomes of green alga of the genus Ostreococcus, which exhibit highly constrained intron sequences through most of the intron-poor genome, but not in one much more intron-dense genomic region. Second, we find evidence that ancestral genomes contained highly variable branch point sequences, similar to more complex modern intron-rich eukaryotic lineages. In addition, ancestral structures are likely to have included polyT tails similar to those in metazoans and plants, which we found in a variety of protist lineages. Intriguingly, intron structure evolution appears to be quite different across lineages experiencing different types of genome reduction: whereas lineages with very few introns tend towards highly regular intronic sequences, lineages with very short introns tend towards highly degenerate sequences. Together, these results attest to the complex nature of ancestral eukaryotic splicing, the qualitatively different evolutionary forces acting on intron structures across modern lineages, and the impressive evolutionary malleability of eukaryotic gene structures.

A single Banana streak virus integration event in the banana genome as the origin of infectious endogenous pararetrovirus

Sequencing of plant nuclear genomes reveals the widespread presence of integrated viral sequences known as endogenous pararetroviruses (EPRVs). Banana is one of the three plant species known to harbor infectious EPRVs. Musa balbisiana carries integrated copies of Banana streak virus (BSV), which are infectious by releasing virions in interspecific hybrids. Here, we analyze the organization of the EPRV of BSV Goldfinger (BSGfV) present in the wild diploid M. balbisiana cv. Pisang Klutuk Wulung (PKW) revealed by the study of Musa bacterial artificial chromosome resources and interspecific genetic cross. cv. PKW contains two similar EPRVs of BSGfV. Genotyping of these integrants and studies of their segregation pattern show an allelic insertion. Despite the fact that integrated BSGfV has undergone extensive rearrangement, both EPRVs contain the full-length viral genome. The high degree of sequence conservation between the integrated and episomal form of the virus indicates a recent integration event; however, only one allele is infectious. Analysis of BSGfV EPRV segregation among an F1 population from an interspecific genetic cross revealed that these EPRV sequences correspond to two alleles originating from a single integration event. We describe here for the first time the full genomic and genetic organization of the two EPRVs of BSGfV present in cv. PKW in response to the challenge facing both scientists and breeders to identify and generate genetic resources free from BSV. We discuss the consequences of this unique host-pathogen interaction in terms of genetic and genomic plant defenses versus strategies of infectious BSGfV EPRVs.

When good transcripts go bad: artifactual RT-PCR 'splicing' and genome analysis

Gene and intron prediction are essential for accurate inferences about genome evolution. Recently, two genome-wide studies searched for recent intron gains in humans, reaching very different conclusions: either of a complete absence of intron gain since early mammalian evolution, or of creation of numerous introns by genomic duplication in repetitive regions. We discuss one possible explanation: the underappreciated phenomenon of "template switching", by which reverse transcriptase may create artifactual splicing-like events in the preparation of cDNA/EST libraries, may cause complications in searches for newly gained introns in repetitive regions. We report large numbers of apparent template switching in transcript sequences from the intron-poor protists Trichomonas vaginalis and Giardia lamblia. Supplementary material for this article can be found on the BioEssays website (http://www.interscience.wiley.com/jpages/0265-9247/suppmat/index.html).

Hsp90n - An accidental product of a fortuitous chromosomal translocation rather than a regular Hsp90 family member of human proteome

Human cells express two isoforms of the Hsp90 protein, called Hsp90alpha and Hsp90beta. Although existence of the third form called Hsp90alphaDeltaN, or Hsp90N was reported in 1998, our investigation, based on the sequence analysis and attempts to reproduce previous results, demonstrate that there is no evidence that Hsp90N gene is present in human genome and no homologs of such a protein are present in other known eukaryotic genomes. We propose that Hsp90N was created as an artifact of a cDNA synthesis or that it is a chimeric protein, being a result of the chromosomal rearrangement that occurred in a single cell line, after this line was established.

The central role of a SNRK2 kinase in sulfur deprivation responses

In the absence of sulfur (S), Chlamydomonas reinhardtii increases the abundance of several transcripts encoding proteins associated with S acquisition and assimilation, conserves S amino acids, and acclimates to suboptimal growth conditions. A positive regulator, SAC1 (for sulfur acclimation protein 1), and a negative regulator, SAC3, were shown to participate in the control of these processes. In this study, we investigated two allelic mutants (ars11 and ars44) affected in a gene encoding a SNRK2 (for SNF1-related protein kinase 2) kinase designated SNRK2.1. Like the sac1 mutant, both snrk2.1 mutants were deficient in the expression of S-responsive genes. Furthermore, the mutant cells bleached more rapidly than wild-type cells during S deprivation, although the phenotypes of ars11 and ars44 were not identical: ars11 exhibited a more severe phenotype than either ars44 or sac1. The phenotypic differences between the ars11 and ars44 mutants reflected distinct alterations of SNRK2.1 mRNA splicing caused by insertion of the marker gene. The ars11 phenotype could be rescued by complementation with SNRK2.1 cDNA. In contrast to the nonepistatic relationship between SAC3 and SAC1, characterization of the sac3 ars11 double mutant showed that SNRK2.1 is epistatic to SAC3. These data reveal the crucial regulatory role of SNRK2.1 in the signaling cascade critical for eliciting S deprivation responses in Chlamydomonas. The phylogenetic relationships and structures of the eight members of the SNRK2 family in Chlamydomonas are discussed.

Enrichment of alternatively spliced isoforms

Most metazoan genes are alternatively spliced, and a large number of alternatively spliced isoforms are likely to be functionally significant and expressed at specific stages of pathogenesis or differentiation. Splicing changes usually only affect a small portion of a gene, and these changes may cause significant mRNA degradation. After RT-PCR, minor variants can form heteroduplexes with the major variants. Affinity purification of these heteroduplexes using immobilized Thermus aquaticus single-stranded DNA-binding protein allows purification of alternative splice forms in a 1:1 ratio, which makes it easy to sequence the rare form. This chapter provides a detailed protocol of the technique I have developed to identify spliced isoforms called enrichment of alternatively spliced isoforms or EASI.

RBM6-RBM5 transcription-induced chimeras are differentially expressed in tumours

Background

Transcription-induced chimerism, a mechanism involving the transcription and intergenic splicing of two consecutive genes, has recently been estimated to account for ~5% of the human transcriptome. Despite this prevalence, the regulation and function of these fused transcripts remains largely uncharacterised.

Results

We identified three novel transcription-induced chimeras resulting from the intergenic splicing of a single RNA transcript incorporating the two neighbouring 3p21.3 tumour suppressor locus genes, RBM6 and RBM5, which encode the RNA Binding Motif protein 6 and RNA Binding Motif protein 5, respectively. Each of the three novel chimeric transcripts lacked exons 3, 6, 20 and 21 of RBM6 and exon 1 of RBM5. Differences between the transcripts were associated with the presence or absence of exon 4, exon 5 and a 17 nucleotide (nt) sequence from intron 10 of RBM6. All three chimeric transcripts incorporated the canonical splice sites from both genes (excluding the 17 nt intron 10 insertion). Differential expression was observed in tumour tissue compared to non-tumour tissue, and amongst tumour types. In breast tumour tissue, chimeric expression was associated with elevated levels of RBM6 and RBM5 mRNA, and increased tumour size. No protein expression was detected by in vitro transcription/translation.

Conclusion

These results suggest that RBM6 mRNA experiences altered co-transcriptional gene regulation in certain cancers. The results also suggest that RBM6-RBM5 transcription-induced chimerism might be a process that is linked to the tumour-associated increased transcriptional activity of the RBM6 gene. It appears that none of the transcription-induced chimeras generates a protein product; however, the novel alternative splicing, which affects putative functional domains within exons 3, 6 and 11 of RBM6, does suggest that the generation of these chimeric transcripts has functional relevance. Finally, the association of chimeric expression with breast tumour size suggests that RBM6-RBM5 chimeric expression may be a potential tumour differentiation marker.

Differential transcription initiation and alternative RNA splicing of Knox7, a class 2 homeobox gene of maize

Knox7, a class 2 homeobox gene has been characterized in maize. A combination of experimental (3'- and 5'-RACE) and bioinformatics approaches supported the idea that Knox7 would be transcribed into two alternative transcripts by differential initiation of transcription. Sequence differences between alternative transcripts, Knox7L the larger and Knox7S the smaller, were confined to their 5' end regions and exon 1 was only found in Knox7L transcripts. Deduced proteins shared the same homeodomain, while an Ala and Ala/Gly rich domain was found only in KNOX7L protein. We hypothesize that KNOX7L and KNOX7S might regulate (differentially) the expression of the same gene(s) by binding competitively to the same cis-acting element(s). Further expression analysis using RT-PCR to amplify cDNA portions corresponding to ORFs of both Knox7 alternative transcripts showed that seven cDNA clones were probably generated by alternative splicing of Knox7L. Alignment of these sequences showed that they are in frame suggesting the existence of the corresponding proteins. Quantitative RT-PCR experiments indicated that Knox7S and Knox7L were expressed in maize embryos during germination. In the same tissue, expression of Knox7S was stimulated by light and ABA and inhibited by GA, two hormones that control germination process.

Expression pattern analysis of transcribed HERV sequences is complicated by ex vivo recombination

Background

The human genome comprises numerous human endogenous retroviruses (HERVs) that formed millions of years ago in ancestral species. A number of loci of the HERV-K(HML-2) family are evolutionarily much younger. A recent study suggested an infectious HERV-K(HML-2) variant in humans and other primates. Isolating such a variant from human individuals would be a significant finding for human biology.

Results

When investigating expression patterns of specific HML-2 proviruses we encountered HERV-K(HML-2) cDNA sequences without proviral homologues in the human genome, named HERV-KX, that could very well support recently suggested infectious HML-2 variants. However, detailed sequence analysis, using the software RECCO, suggested that HERV-KX sequences were produced by recombination, possibly arising ex vivo, between transcripts from different HML-2 proviral loci.

Conclusion

As RT-PCR probably will be instrumental for isolating an infectious HERV-K(HML-2) variant, generation of "new" HERV-K(HML-2) sequences by ex vivo recombination seems inevitable. Further complicated by an unknown amount of allelic sequence variation in HERV-K(HML-2) proviruses, newly identified HERV-K(HML-2) variants should be interpreted very cautiously.

EASI--enrichment of alternatively spliced isoforms

Alternative splicing produces more than one protein from the majority of genes and the rarer forms can have dominant functions. Instability of alternative transcripts can also hinder the study of regulation of gene expression by alternative splicing. To investigate the true extent of alternative splicing we have developed a simple method of enriching alternatively spliced isoforms (EASI) from PCRs using beads charged with Thermus aquaticus single-stranded DNA-binding protein (T.Aq ssb). This directly purifies the single-stranded regions of heteroduplexes between alternative splices formed in the PCR, enabling direct sequencing of all the rare alternative splice forms of any gene. As a proof of principle the alternative transcripts of three tumour suppressor genes, TP53, MLH1 and MSH2, were isolated from testis cDNA. These contain missing exons, cryptic splice sites or include completely novel exons. EASI beads are stable for months in the fridge and can be easily combined with standard protocols to speed the cloning of novel transcripts.

Comprehensive splice-site analysis using comparative genomics

We have collected over half a million splice sites from five species-Homo sapiens, Mus musculus, Drosophila melanogaster, Caenorhabditis elegans and Arabidopsis thaliana-and classified them into four subtypes: U2-type GT-AG and GC-AG and U12-type GT-AG and AT-AC. We have also found new examples of rare splice-site categories, such as U12-type introns without canonical borders, and U2-dependent AT-AC introns. The splice-site sequences and several tools to explore them are available on a public website (SpliceRack). For the U12-type introns, we find several features conserved across species, as well as a clustering of these introns on genes. Using the information content of the splice-site motifs, and the phylogenetic distance between them, we identify: (i) a higher degree of conservation in the exonic portion of the U2-type splice sites in more complex organisms; (ii) conservation of exonic nucleotides for U12-type splice sites; (iii) divergent evolution of C.elegans 3' splice sites (3'ss) and (iv) distinct evolutionary histories of 5' and 3'ss. Our study proves that the identification of broad patterns in naturally-occurring splice sites, through the analysis of genomic datasets, provides mechanistic and evolutionary insights into pre-mRNA splicing.