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Full-length autonomous transposable elements are preferentially targeted by expression-dependent forms of RNA-directed DNA methylation

ABSTRACT: The diversity of small RNA-directed DNA methylation (RdDM) mechanisms have been underestimated due to the nearly complete transcriptional silencing of transposable elements (TEs) in the wild-type reference strains of Arabidopsis thaliana. In plants mutant for the SWI/SNF histone remodeler DDM1, TEs are globally activated due to loss of genome wide heterochromatin condensation. Transcriptionally activated TEs go through additional non-canonical forms of RdDM that are dependent on RNA Polymerase II expression. However, the global targets of the non-canonical RdDM pathway have not been explored. In an attempt to identify and contrast the targets of canonical and expression-dependent non-canonical RdDM, we performed MethylC-seq of genome-wide DNA methylation patterns from several RdDM mutants in either the TE-silent or the TE-active (ddm1) contexts.

ORGANISM(S): Arabidopsis thaliana

PROVIDER: GSE79746 | GEO | 2016/07/22

SECONDARY ACCESSION(S): PRJNA316982

REPOSITORIES: GEO

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Project description:Background: Transposable elements (TEs) represent a substantial fraction of the genomes, playing a major role in evolution, as sources of genetic variability. To fully appraise the role of TE in the acquisition of genetic novelty in genome evolution, we must also consider the impact of their own transcriptional activity. Results: We studied impact of TE transcriptional activity on gene using high-throughput RNA-Seq sequencing in Drosophila melanogaster. TEs, which turn out to be expressed in euchromatin as well as in heterochromatin, interact with genes at different levels. The observed transcription from TEs involve canonical or non-canonical transcription start sites (TSSs) distributed along their sequence. We also find evidences for potential bidirectional transcription from the TE promoter regions where the antisense transcript is co-opted by the host genome as TSSs of a gene. We found that active TEs seem to accumulate in the 5' upstream regions of the genes, and possibly provide an alternative transcript of the nearby gene. Indeed, predominantly, the TE transcript is collinear and overlapping the gene. Apart from the 5' upstream regions, we also found that most active TEs are transcribed on the gene transcript strand. Conversely, few transcripts from TE are anti sense with respect to the gene. This suggests that they have a disruptive action and are counter-selected. The only exceptions are for TEs located into introns, where they could provide another complex way of gene regulation, and in the 3' downstream region, where other mechanisms akin to siRNAs could take place. Finally, we noted several cases where the cryptic TSS is located on TE fragments corresponding to a low complexity sequence. Frequently these TE fragments appear to be over-represented when close to genes, suggesting a possible selected role. Conclusion: Altogether, these results suggest that active transposable elements influence host gene transcription. It is likely that some features of transposable elements have been exaptated in order to enrich the genes repertoire by opening routes to sub- or neo-functionalization. Examination of the transcription produced by transposable elements in D. melanogaster

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Full-length autonomous transposable elements are preferentially targeted by expression-dependent forms of RNA-directed DNA methylation

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