Abstract
BackgroundThe activity of transposable elements can be regulated by different means. DNA CpG methylation is known to decrease or inhibit transpositional activity of diverse transposons. However, very surprisingly, it was previously shown that CpG methylation of the Sleeping Beauty (SB) transposon significantly enhanced transposition in mouse embryonic stem cells.ResultsIn order to investigate the unexpected response of SB transposition to CpG methylation, related transposons from the Tc1/mariner superfamily, that is, Tc1, Himar1, Hsmar1, Frog Prince (FP) and Minos were tested to see how transposition was affected by CpG methylation. A significant increase of >20-fold in transposition of SB, FP and Minos was seen, whereas Tc1, Himar1 and Hsmar1 showed no difference in transposition upon CpG-methylation. The terminal inverted repeats (TIRs) of the SB, FP and Minos elements share a common structure, in which each TIR contains two functionally important binding sites for the transposase (termed the IR/DR structure). The group of IR/DR elements showed increased excision after CpG methylation compared to untreated transposon donor plasmids. We found that de novo CpG methylation is not required for transposition. A mutated FP donor plasmid with depleted CpG sites in both TIRs was as efficient in transposition as the wild-type transposon, indicating that CpG sites inside the TIRs are not responsible for altered binding of factors potentially modulating transposition. By using an in vivo one-hybrid DNA-binding assay in cultured human cells we found that CpG methylation had no appreciable effect on the affinity of SB transposase to its binding sites. However, chromatin immunoprecipitation indicated that CpG-methylated transposon donor plasmids are associated with a condensed chromatin structure characterized by trimethylated histone H3K9. Finally, DNA compaction by protamine was found to enhance SB transposition.ConclusionsWe have shown that DNA CpG methylation upregulates transposition of IR/DR elements in the Tc1/mariner superfamily. CpG methylation provokes the formation of a tight chromatin structure at the transposon DNA, likely aiding the formation of a catalytically active complex by facilitating synapsis of sites bound by the transposase.
Highlights
The activity of transposable elements can be regulated by different means
In order to extend these observations to other Tc1/mariner transposons, the effect of CpG methylation on Tc1 and the Tc1-like elements Frog Prince (FP) and Minos, and the mariner-like elements Himar1 and Hsmar1 was investigated
We generated transposon donor plasmids containing the respective Terminal inverted repeat (TIR) flanking a neomycinresistance gene trap cassette cloned into identical plasmid backbones
Summary
DNA CpG methylation is known to decrease or inhibit transpositional activity of diverse transposons. One class of repetitive sequences found in heterochromatic regions of different genomes are TEs, and it is believed that the accumulation of transposable DNA sequences in heterochromatic regions provides a safe place, where the deleterious potential of these elements can be kept on a leash [14]. There is a strong correlation between chromatin structure and the activity of TEs. For example, recruiting transposable DNAs into heterochromatic regions may provide efficient silencing of transcription of element-encoded proteins, and provides genome stability. In addition to its repressive function on transcription, heterochromatin exerts a repressive influence on recombination [15,16]; the containment of repeated sequences in heterochromatic regions may prevent irregular recombination and genome instability
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