Revisiting the evolution of mouse LINE-1 in the genomic era

LINE-1 (L1) is the dominant category of transposable elements in placental mammals. L1 has significantly affected the size and structure of all mammalian genomes and understanding the nature of the interactions between L1 and its mammalian host remains a question of crucial importance in comparative genomics. For this reason, much attention has been dedicated to the evolution of L1. Among the most studied elements is the mouse L1 which has been the subject of a number of studies in the 1980s and 1990s. These seminal studies, performed in the pre-genomic era when only a limited number of L1 sequences were available, have significantly improved our understanding of L1 evolution. Yet, no comprehensive study on the evolution of L1 in mouse has been performed since the completion of this genome sequence.

Using the Genome Parsing Suite we performed the first evolutionary analysis of mouse L1 over the entire length of the element. This analysis indicates that the mouse L1 has recruited novel 5'UTR sequences more frequently than previously thought and that the simultaneous activity of non-homologous promoters seems to be one of the conditions for the co-existence of multiple L1 families or lineages. In addition the exchange of genetic information between L1 families is not limited to the 5'UTR as evidence of inter-family recombination was observed in ORF1, ORF2, and the 3'UTR. In contrast to the human L1, there was little evidence of rapid amino-acid replacement in the coiled-coil of ORF1, although this region is structurally unstable. We propose that the structural instability of the coiled-coil domain might be adaptive and that structural changes in this region are selectively equivalent to the rapid evolution at the amino-acid level reported in the human lineage.

The pattern of evolution of L1 in mouse shows some similarity with human suggesting that the nature of the interactions between L1 and its host might be similar in these two species. Yet, some notable differences, particularly in the evolution of ORF1, suggest that the molecular mechanisms involved in host-L1 interactions might be different in these two species.