Exonization of active mouse LIs: a driver of transcriptome evolution?

Background: Long interspersed nuclear elements (LINE-Is, LIs) have been recently implicated in the regulation of mammalian transcriptomes. Results: Here, we show that members of the three active mouse LI subfamilies (A, G(F) and T-F) contain, in addition to those on their sense strands, conserved functional splice sites on their antisense strands, which trigger multiple exonization events. The latter is particularly intriguing in the light of the strong antisense orientation bias of intronic LIs, implying that the toleration of antisense insertions results in an increased potential for exonization. Conclusion: In a genome- wide analysis, we have uncovered evidence suggesting that the mobility of the large number of retrotransposition-competent mouse LIs (similar to 2400 potentially active LIs in NCBIm35) has significant potential to shape the mouse transcriptome by continuously generating insertions into transcriptional units.