Category Archives: mobile DNA

The Transposon/piRNA/Chromatin Nexus

Close observation of chromatin states at piRNA-silenced genomic loci demonstrates the power of transposons to change native gene expression.

As reviewed in an earlier post, the Drosophila Piwi/piRNA transposon silencing pathway can be divided into two facets; a complex pathway operating in the germline centred on the Piwi-family argonautes Aubergine and AGO3 localised in peri-nuclear nuage, and a linear pathway operational in the somatic follicle cells. In this linear pathway, piRNAs derived from uni-directional piRNA clusters such as flamenco target Piwi to mediate silencing of a limited subset of retrotransposons. Unlike Aub and AGO3, Piwi is localised to the nucleus, leading to speculation that rather than silencing transposons post-transcriptionally by ‘slicing’ their transcripts, it may act at the transcriptional level. There are many precedents in other organisms for argonautes mediating transcriptional silencing via interactions with chromatin modification and DNA methylation pathways. However, whether one of these silencing modes is employed by Drosophila Piwi was unresolved. A new paper from the lab of Julius Brennecke, generally analysing the linear piRNA pathway active in a cell line derived from the somatic follicle cells surrounding the oocyte (OSC cells) includes important findings for a number of aspects of Piwi-mediated transposon silencing leading to insights on the wider genomic ecology of transposon insertions.

In the first section of the paper, Sienski et al. demonstrate that Maelstrom (Mael), a protein containing putative RNA and DNA binding domains, expressed in both cytoplasm and nuclei and previously implicated in a number of Piwi-pathway effects, acts downstream of Piwi to effect TE silencing. Silencing requires the nuclear localisation of both Piwi and Mael. Further, mutation of the residues necessary for ‘slicer’ activity in Piwi did not de-repress TEs, suggesting a different mechanism for Piwi-mediated silencing.

Sienski et al. go on to marshal three different high-throughput techniques to show that Piwi mediates gene silencing at the transcriptional level. Knocking down (KD) the expression of Piwi pathway factors (piwi, mael) in OSC cells they determined the set of repressed transposable elements (TEs) by comparing RNA levels (RNA-seq). Changes in the steady-state RNA levels were highly correlated with transcription rate as monitored by RNA polymerase II occupancy (ChIP-seq) and levels of nascent RNAs (GRO-seq). Judging by how closely correlated derepression of TEs was to transcription rate, it seems unlikely that the linear piRNA pathway active in follicle cells acts post-transcriptionally at all.

Reasoning that Piwi-mediated transcriptional gene silencing may involve chromatin modification, Sienski et al. profiled the distribution of the repressive histone mark H3K9me3 in OSCs after piwi or mael knockdown. H3K9me3 levels at transposable elements known to be repressed by the piRNA pathway were significantly reduced in the absence of Piwi (and to a lesser extent Mael). This data was from across the genome irrespective of whether the TE was inserted into heterochromatic or euchromatic regions. To negate general effects associated with heterochromatin, the authors looked more closely at TE insertions within euchromatic regions.

Approximate sketch of the patterns of RNA pol II occupancy (ie Transcription), and H3K9me3 at the mdg1 locus after piwi or mael knockdown and normally in control.

Approximate sketch of the patterns of RNA pol II occupancy (ie Transcription), and H3K9me3 at the mdg1 locus after piwi or mael knockdown and normally in control.

At a specific euchromatic insertion of the retrotransposon mdg1, they observed that upon either piwi KD or mael KD, transcription downstream of the insertion strongly increased. However, although this transcriptional bleeding into the surrounding area was similar upon TE derepression due to either piwi KD or mael KD, the pattern of H3K9me3 was very different. Normally this mdg1 insertion displays H3K9me3 in the surrounding 12kb, peaking at the insertion site. This was strongly reduced in piwi KD cells, but in mael KD, H3K9me3 was moderately reduced at the insertion site but had actually spread further downstream (see figure). Similar patterns were observed at nearly all euchromatic mdg1 insertions, as well as other TEs known to be targeted by the linear piRNA pathway active in OSC cells.

Strikingly, most euchromatic H3K9me3 peaks were sensitive to piwi knockdown, whilst 88% of H3K9me3 peaks were found within 5Kb of TE insertions. Piwi-mediated transposon silencing therefore seems to be the main trigger for H3K9 trimethylation in euchromatin.

This transposon silencing mechanism appears to have a major impact on native genes upon TE insertion in their vicinity. An insertion of the retrotransposon gypsy into the first intron of the expanded (ex) gene serves as paradigm for these effects. In OSC cells, the gypsy insertion triggered H3K9me3 spreading into the surrounding 10-12Kb. In control cells RNA polymerase II occupancy was observable at the ex transcription start site (TSS) but weak. Upon piwi or mael knockdown, transcription from the ex TSS was massively increased. As in the earlier mdg1 example, H3K9me3 levels were greatly reduced upon piwi KD but not in mael KD cells. Sienski et al. observed similar effects on the transcription of 28 more genes with nearby TE insertions in OSC cells.

This data has a number of ramifications speaking of a complex interplay between transcription, the establishment and maintenance of repressive chromatin states and the Piwi pathway. Firstly, H3K9me3 considered a transcriptionally repressive histone mark is compatible with transcription. In fact, based on it’s pattern in mael KD cells, the authors propose that downstream transcriptional bleeding leads to the spread of H3K9me3. Further, although H3K9me3 has an integral role in Piwi-mediated silencing, it is not the final silencing mark. H3K9 trimethylation is downstream of Piwi action, but is either upstream or acts in parallel to Mael, which mediates an unknown silencing step crucial to Piwi transcriptional gene silencing.

Importantly, this paper has demonstrated the impact that TE insertion and subsequent piRNA pathway transcriptional repression can have on native gene expression. There are two different modes in which the inactivation of Piwi-mediated TE silencing can lead to the transcriptional activation of these loci. Firstly, the spreading of repressive chromatin marks at transposons can suppress RNA polymerase II access to the genes promoter. Alleviation of TE repression hence leads to (re-)activation of gene expression. Conversely, as TEs (especially the long terminal repeats of some retrotransposons) can serve as promoters, the loss of their repressed chromatin state upon piRNA pathway loss, can activate transcription of downstream regions. Although both these modes lead to transcriptional activation after Piwi pathway loss, they demonstrate that transposon insertion can either activate or repress transcription within relatively extensive genomic surroundings. This underscores the scope for transposons to act as regulatory elements, or to produce new chimerical transcripts and hence potential new genes.

These experiments were mainly performed in one cell type that only partially reflects the activity of what is already a subset of piwi/piRNA action during Drosophila oogenesis.  Piwi and Mael are also active in the nurse cells and oocyte, and this paper suggests that they have similar roles within the context of the expanded piRNA pathways active in the germline. It will be interesting to integrate this nuclear-localised transcriptional-silencing aspect of piRNA silencing into the context of ping-pong amplification and bi-directional piRNA cluster transcripts. Further, do these Piwi-mediated chromatin effects in the germline impact on the transcriptional status of TEs and genes later in somatic development? And if not, do other systems have equivalent activity?

This paper underlines again the importance of the arms race between mobile genetic elements and genomic immune systems such as the piRNA pathway on the wider genomic regulatory context. This contest is being observed to have shaped so many aspects of genome organisation throughout evolution that it sometimes becomes hard to differentiate parasitism from regulation. It is clear however, that to understand the evolutionary impact of mobile elements we must also understand the import of the various epigenetic mechanisms controlling their spread. The minutiae of these mechanisms with regard to their targets, plasticity, adaptability, heritability – often different from organism to organism – has major evolutionary significance. Evolution works differently depending on these mechanisms.

Sienski, G., Dönertas, D., & Brennecke, J. (2012). Transcriptional Silencing of Transposons by Piwi and Maelstrom and Its Impact on Chromatin State and Gene Expression Cell, 151 (5), 964-980 DOI: 10.1016/j.cell.2012.10.040

A chimeric fusion of RNA and DNA viruses.

The discovery of a new family of viruses leads to speculations on possible modes recombination between RNA and DNA viruses.

The virosphere can be divided into three major classes; viruses with DNA genomes, retroviruses that reverse-transcribe their RNA genome into DNA during their lifecycle, and RNA-only viruses that don’t require DNA intermediates to replicate. In fact, viruses use all sorts of different permutations of genetic material; double-stranded RNA, single-stranded RNA (either negative or positive strand), dsDNA and ssDNA. Viruses evolve notoriously quickly and lateral gene transfer between them is rampant. However, gene transfer has most commonly occurred between closely related viruses or between those with similar replication mechanisms. A recent paper has reported the discovery of a new family of viruses that appear to have arisen via lateral gene transfer between a (non-retroid) +ve single-stranded RNA virus and a ssDNA virus.

Diemer and Stedman discovered the new virus whilst investigating viral diversity in a geothermal lake in California. Boiling Springs Lake is an acidic, high temperature lake with a purely microbial ecosystem composed of archaea, bacteria, and some single cell eukaryotes. Using a metagenomics approach (ie. large-scale sequencing  of environmental DNA from a virus particle sized fraction), they discovered the strange juxtaposition of a capsid protein (CP) gene related to those from the ssRNA plant-infecting Tombusviridae, with a rolling-circle replicase (Rep) gene most similar to those from the circular ssDNA-containing Circoviridae. Using primers designed against CP they confirmed the genome sequence of this putative virus, finding that it consisted of a single-stranded circular DNA containing 4 ORFs. ORFs 3 and 4 are of unknown function and unrelated to known genes. The virus contains a stem loop structure upstream of the Rep gene similar to those that serve as replication origins in other Circoviruses. Thanks to the chimeric origin of the Rep and CP genes, the authors termed it RNA-DNA hybrid virus (RDHV). This term is slightly open to misinterpretation as it could suggest that both molecules are actually encoding its’ genome, but to be clear this is a circular ssDNA virus whose capsid protein is derived from ssRNA viruses.

Organisation of RDHV. Note that ORFs 3 and 4 are not equivalent to those of Tombusviruses, and RDHV is twice the size of other Circoviruses.

Scanning databases of environmental sequence, the researchers found three other instances of homologous CP and Rep sequences arranged in the same configuration, two from global ocean surveys and one from the Sargasso Sea. This shows that RDHV defines a new family of viruses that are common in marine environments and could be more widespread. As CP and Rep are still highly similar to their sibling genes, it appears that the LGT event underlying the evolution of this new family occurred quite recently.

How did recombination occur between a non-retrovirus ssRNA virus and a DNA virus? A number of genes derived from non-retroid RNA viruses have been found in eukaryotic genomes, so perhaps this type of exchange is not as strange or rare as it may seem. The most likely scenario involves the RNA gene being converted into DNA by reverse transcription, followed by DNA-DNA recombination. As reverse transcriptase is not encoded by either virus, it could have been supplied in trans by retrotransposons, group II introns, or retroviruses within a common host cell. This brings us to the problem of metagenomic studies; they have amazing power to identify novel viruses and organisms, but yield very little information on the biology of what is found. In this case of RDHV and it’s family we do not know what their hosts are, don’t know the morphology of the viruses, and don’t know about the functions of half it’s 4 gene genome. I’m not sure how quickly these questions will be answered. Nevertheless, this study shows that amazing diversity is still out there being found, and yields insight into mechanisms underlying virus evolution – possibly in the deep past as well as more recently.

Diemer, G., & Stedman, K. (2012). A novel virus genome discovered in an extreme environment suggests recombination between unrelated groups of RNA and DNA viruses Biology Direct, 7 (1) DOI: 10.1186/1745-6150-7-13

Beating a Toxin-Antitoxin System; Evading Suicide

Bacteria have evolved many different systems to evade viral predation. One strategy, abortive infection (Abi), involves altruistic suicide. Mediated by a toxin-antitoxin (TA) system, the suicide of the infected cell protects the clonal bacterial population by preventing the spread of replicated bacteriophage. A new paper in Plos Genetics has discovered a molecular mimicry-based strategy that allows phage to escape abortive infection.

Toxin-antitoxin systems are widespread prokaryotic genetic elements found on both plasmids and bacterial chromosomes. Encoding a relatively long-lived toxin and a more labile antitoxin expressed from a single bi-cistronic operon, they were originally characterised as ‘addiction modules’. In the event that a plasmid expressing a TA system fails to be inherited by a daughter cell, the absence of antitoxin allows the persisting toxin to kill the cell – post-segregational killing. These attributes as ‘selfish elements’ made it slightly surprising that so many TA systems have been found encoded on bacterial chromosomes themselves. I’ve previously written about an example of one such TA system’s activity in mediating a stress response in E. coli, and they’ve also been implicated in the formation of antibiotic resisting ‘persister’ cells.

TA systems have been classified into three different classes defined by the level of the molecular interaction between their two components. In type I systems, translation of the toxin is prevented by an antisense RNA antitoxin binding to its’ transcript, whilst in type II systems both partners are proteins. Most recently, type III TA systems have been characterised in which the toxin is neutralised by binding of an RNA antitoxin. Examples of all three varieties have been found to protect bacteria from phage infection via abortive infection; phage replication disrupts the normal cellular transcriptional program, interrupting antitoxin production and hence leading to cell death.

The first type III TA system to be characterised, ToxIN, was found on plasmids in the phytopathogen, Pectobacterium atrosepticum (Pba), and shown to inhibit the propagation of multiple different bacteriophage. ToxN is an endoribonuclease, whilst the antitoxin ToxI, is a 36nt RNA structured as a ‘pseudoknot’. The partners combine into a hetero-hexameric structure composed of 3 ToxN molecules and 3 ToxI pseudoknots.

Blower et al. have discovered that phage can evade the Abi system by producing molecular mimics of the ToxI RNA. The lytic bacteriophage ΦTE, normally fails to infect Pba carrying a toxIN-containing plasmid. At low frequency however, new phage strains emerge capable of evading the Abi system. Upon sequencing the genomes of these ‘escape strains’, the researchers discovered that they all contained sequence expansions at one specific locus. The toxI locus contains 5.5 repeats of the 36nt RNA pseudoknot-encoding sequence. The ‘escape locus’ from the phage normally encoded 1.5 repeats of a pseudo-ToxI sequence. In all the escape strains this repeat had been expanded so that it contained either 4.5 or 5.5 repeats. These expansions had probably arisen due to strand-slippage during phage replication. In one escape strain homologous recombination had occurred between the phage pseudo-ToxI and the endogenous toxI; the phage had effectively hijacked a normal antitoxin- encoding gene.

The 1.5 repeat pseudo-ToxI could not inhibit Abi (as the sequence was out of phase it did not actually encode a functional psudoknot). However, the repeat expansions had allowed the phage to make an antitoxin mimic that protected them from the TA system and hence Abi.

ΦTE is capable of generalised transduction – the ability to package and transfer chromosomal and plasmid DNA from its’ host and transfer it during infection. Blower et al. showed that one of the ΦTE escape strains is able to transduce the plasmid encoded ToxIN – a case of a bacteriophage horizontally transferring an anti-phage defence mechanism. This brings into focus the complex evolutionary dynamics operating between the three different genetic entities being studied; the bacterial cell, the plasmid encoding the TA system, and the bacteriophage evading it and potentially propagating it. From the selfish viewpoint of the TA module what’s best, preventing the spread of the phage or being disseminated by it? These speculations aren’t about to be easily answered, however, it is an interesting way to analyse further examples of similar systems.

Blower, T., Evans, T., Przybilski, R., Fineran, P., & Salmond, G. (2012). Viral Evasion of a Bacterial Suicide System by RNA–Based Molecular Mimicry Enables Infectious Altruism PLoS Genetics, 8 (10) DOI: 10.1371/journal.pgen.1003023

Chromosomal Flip-Flop

A study describes how phenotypic switching in Staphylococcus aureus is caused by a reversible large-scale genomic inversion.

Clonal bacterial populations often display various phenotypes. This diversity is most obviously identifiable as colony variation. Many different bacterial genera display ‘small colony variants’ (SCVs), the occurrence of which is generally attributable to deficiencies in various metabolic pathways.

Cui et al have characterised an STV strain of Staphylococcus aureus which reverts to a normal colony variant (NCV) at a frequency of 1-3 in a 1000. Interestingly the NCV progeny revert back to SCV in 1-10% of cases. This frequent bi-directional reversion was stably maintained at these ratios; homogeneous colony populations could never be isolated.

The small colony variant displayed some important phenotypic differences to the NCV. As well as slow growth and less pigmentation, it was susceptible to β-lactam antibiotics whilst the NCV was not. The authors identified over a hundred genes were differentially expressed between the two variants, and that their susceptibilities to many chemicals were different.

Diagram showing reversible genomic inversion forms caused by homologous recombination at inverted repeat regions (break points. BPs)

When Cui et al. sequenced the genomes of the two variants, they discovered that nearly half of the genome (1.26 Mb of 2.87 Mb) was differenttly aligned. This ‘X-shaped’ chromosome inversion occurred between two oppositely oriented pathogenicity islands, symmetrically opposite each other on the chromosome with respect to the replication axis. Each pathogenicity island contained two copies of an identical 3,638bp long sequence. It appears that homologous recombination can occur at these sites and generate the genomic inversion. This is in agreement with experiments in which the authors altered levels of the key recombination regulatory protein RecA; finding that they could increase the rate of reversion with higher recA expression.

The chromosomal flip-flopping therefore regulates the maintenance of two different S. aureus phenotypic variants. The two forms have different advantages and disadvantages. The original SCV strain isolated from a patient suffering persistent reinfection of a surgical site. It appears that the SCV may facilitate immune evasion, whilst the NCV has higher antibiotic resistance. Maintaining a balance between the two variants within the S. aureus population therefore functions as an evolutionarily useful bet-hedging strategy.

This type of flexible genome organisation serving as a self-organising regulatory mechanism for the maintenance of a bi-stable heterogeneous cell population may well be a more wide-spread bacterial evolutionary strategy.

Cui L, Neoh HM, Iwamoto A, & Hiramatsu K (2012). Coordinated phenotype switching with large-scale chromosome flip-flop inversion observed in bacteria. Proceedings of the National Academy of Sciences of the United States of America, 109 (25) PMID: 22645353

Expanding the Conjugative Realm

A recent paper demonstrates that a broader range of plasmids can be transferred by conjugation than previously thought.

Integrative and conjugative elements (ICEs, aka conjugative transposons) are a variety of bacterial mobile element generally found integrated into the host genome, but capable of excision and transfer to other cells via conjugation. I’ve previously written a short review of some of their key features, which may serve as a helpful introduction to this post. As well as transferring themselves between cells, ICEs and conjugative plasmids can mobilise other DNA elements, such as integrative mobilizable elements (IMEs) and mobilizable plasmids, that aren’t independently capable of self-transfer.

The conjugative transfer of any of these elements generally rests upon the generation of a single-stranded DNA molecule from the circular dsDNA mobile genetic element. The ssDNA is formed by a mobilising relaxase (Mob) nicking the circular DNA at an origin of transfer (oriT) sequence, followed by the unwinding of the strands by a host-derived helicase. Mob covalently binds the nicked end forming the ‘relaxosome’. A coupling protein is responsible for targeting the relaxosome to the conjugative apparatus (a type 4 secretion system, T4SS).

Plasmids that are incapable of self-transfer, but can be mobilised in trans by conjugative elements, generally encode their own mobilising relaxase and a cognate oriT site. These functions are separate from their replication system, which does however consist of similar components; a replication relaxase (Rep) which nicks and binds to an origin of replication (ori).

Lee et al. have discovered that three plasmids, which had been classified as non-mobilisable due to their lack of Mob/oriT functions, can in fact be transferred between Bacillus subtilis cells by the conjugation system of an ICE, ICEBs1. The three plasmids’ (pC194, pBS42, and pHP13) transfer required the conjugation machinery and coupling enzyme (ConQ) of ICEBs1, but was not dependent on it’s relaxase (NicK). Transfer could occur concomitantly with that of ICEBs1 or in it’s absence; showing that transfer did not act in cis due to integration of the plasmid into ICEBs1.

The authors found that, in the case of pBS42, it’s replicative relaxase was necessary for mobilisation. They therefore think it likely that in all three cases the Rep/ori system is also capable of mediating mobilisation functions. This blurring of the distinctions between Rep/ori and Mob/oriT systems has important ramifications. It opens up the possibility that many rolling-circle replicating plasmids that had been classed as non-mobilisable can in fact be transferred between cells via conjugation.

From an evolutionary perspective, these findings are important for understanding the persistence of plasmids in bacterial populations. Non-mobilisable plasmids would only be maintained in the population because benefits conferred on their hosts. If however many more ‘non-mobilisable’ plasmids can be disseminated by horizontal transfer, their persistence can be better explained. This study therefore expands the importance of conjugation in understanding bacterial evolution. Future studies will need to analyse the extent of interactions between coupling proteins and replication relaxases to better gauge the trans-mobilisation of genetic elements, and re-classify the mobility status of many plasmids.

Lee CA, Thomas J, & Grossman AD (2012). The Bacillus subtilis conjugative transposon ICEBs1 mobilizes plasmids lacking dedicated mobilization functions. Journal of bacteriology, 194 (12), 3165-72 PMID: 22505685

Trans-Extremophile Para-Sex

Cells of Haloferax volcanii showing their cup-like morphology

More than twenty years ago, an extra-ordinary mode of para-sexual genetic exchange was found to occur between cells of the halophilic archaeon Haloferax volcanii, an extremophile isolated from the Dead Sea. This process involves first, cell fusion producing heterodiploid cells. In this state, parental chromosomes can recombine, hence producing novel hybrid daughter cells after cell separation.

Haloferax mediterranei

Naor et al. have now shown that this mechanism of genetic exchange can also occur between cells of H. volcanii, and those of a related species – Haloferax mediterranei. H. mediterranei was originally isolated from a saltern near Alicante in Spain, and shares on average 86.6% sequence identity in protein-coding genes with H. volcanii. Assaying for chromosomal markers, it was found that interspecies cell fusion occurs at a frequency of 4.2 x 10-5, less than one order of magnitude less frequently than between H. volcanii cells (1.0 x 10-4). Of intraspecies fusions, 62% resulted in genetic recombination, whereas only 8% of interspecies fusions did. Although these figures sound quite small, in comparison to modes of genetic exchange between bacterial species, this inter-species genetic interchange occurred  a few orders of magnitude more efficiently.

Analysing a number of hybrid genomes, Naor et al. found that recombinant fragments ranged in size between 310 kb and 530 kb – huge genomic portions, with respect to most known bacterial horizontal genetic transfer mechanisms. Recombination occurs at areas higher sequence identity, such as transfer RNA genes. As these hybrids were isolated by selection for a specific genetic marker, it is quite conceivable, that higher proportions of the genomes can be transferred by this process.

This form of genetic exchange has important implications for archaeal speciation and evolution. High rates of recombination can act as a homogenising force – unlinking alleles faster than genes can diversify. The authors therefore suggest that geographical isolation may be the primary force in archaeal speciation. Further experiments testing the efficiency of this parasexual genetic exchange mechanism between other Haloferax species, and more information on halophilic archaeal ecology will help to clarify these issues. It will also be interesting to examine more hybrid genomes derived from other selection experiments to find out just how extensive interspecies recombination can be and whether there are any directional biases in transfer.

Naor A, Lapierre P, Mevarech M, Papke RT, & Gophna U (2012). Low species barriers in halophilic archaea and the formation of recombinant hybrids. Current biology : CB, 22 (15), 1444-8 PMID: 22748314

The Birth of Introns

Eukaryotic genes are composed of exons and introns. Introns are non-coding sequences that separate the coding exons, and are spliced out of the pre-messenger RNA after transcription. This modular structure of eukaryotic genes allows alternative splicing, by which single genes can encode multiple isoforms of proteins, hence widening the diversity of the proteome. Introns also have important roles in genetic regulation; for instance as sites of enhancers, and by encoding microRNAs.

Intron position is often conserved between orthologous eukaryotic genes showing that spliceosomal introns originated early in eukaryotic evolution. However, it has been difficult to explain the mechanisms of intron loss, and especially, gain that have maintained a high number of introns in present day eukaryotic genomes. Current models suggest that introns should be being lost faster than they are gained. However, studies in organisms such as the urochordate, Oikopleura dioca, and the green alga, Micromonas pusilla, have shown extensive recent intron gains. Interestingly, the study of the Micromonas genome discovered a form of intronic repeat sequence that ‘extended nearly to donor and acceptor sites, and lacked known TE (transposable element) characteristics’. These sequences were termed ‘Introner elements’. A new study, forthcoming in Current Biology, has discovered and characterised something similar in various fungal clades.

Burgt et al. found numerous introns with near-identical sequences in the Dothidiomycete fungus Cladosporium fulvum. They then widened their analysis to search for similar introns in the ‘intronomes’ of 23 other species of fungi, and found large sets of near-identical introns in 6 different species. Phylogenetic analyses of these ‘introner-like elements’ (ILEs) showed that they could be grouped into related clusters, and that in turn the clusters were related to each other, indicating that all the ILE clusters were derived from a single ancestral element.

Analysis of the molecular structure of the Introner-like elements showed that they contained all the distinguishing features of normal spliceosomal introns, such as splice acceptor and donor sites, and branch point sequences. ILEs were longer than normal introns, and were found to fold into more stable secondary structures. Burgt et al. suggest that these predicted stable secondary structures are likely to have important functions, as they observed compensatory mutations that conserve secondary structure between related ILEs.

Analysing intron gain in the 6 species of fungi in which they found ILEs, Burgt et al find that ILEs account for the majority of recent gains. In closely related sister species that diverged within the last 22,000 years ILEs account for 90% of intron gains, but this figure rapidly drops off for older divergences. This leads Burgt et al. to consider that most intron gains are due to ILE multiplication, with rapid degeneration meaning that ILE identification becomes progressively more difficult.

Introner-like elements therefore appear to be mobile elements that can in some way transpose to new sites leading to intron gain. Just what mechanism is employed in this process is far from clear. Many different mechanisms for intron gain have been proposed but as yet there is little experimental evidence demonstrating that they occur in vivo. These include Intron transposition, in which an intron transposes to a new position in a transcript, which is then reverse transcribed and recombined into the original gene; Transposon insertion in which a transposon becomes a spliceable intron; Intronisation in which exons are converted into intron by accumulated mutation; and other ideas based on genetic duplications and errors during repair processes. Burgt et al think that the most likely mechanism for ILEs is a process by which introns are reverse spliced directly into the genome and then reverse transcribed. It will be interesting to see whether ILE transposition can be observed in vivo and figure out just what mechanism of intron generation is employed.

Interestingly, introner-like elements differ from the introner elements found in Micromonas in important ways. Introner elements were found within introns rather than being the whole intron, and lacked the interesting secondary structures observed in ILEs. Along with the author’s inability to find ILEs in other clades, this suggests that ILEs may not be a very widespread mechanism of intron multiplication. However Burgt et al. disagree, and reckon that ILEs could potentially be an ancestral mechanism for intron gain.

van der Burgt, A., Severing, E., de Wit, P., & Collemare, J. (2012). Birth of New Spliceosomal Introns in Fungi by Multiplication of Introner-like Elements Current Biology DOI: 10.1016/j.cub.2012.05.011