Ogre elements — A distinct group of plant Ty3/gypsy-like retrotransposons

Ogre elements are a group of LTR retrotransposons recently discovered in legume plants, where they constitute almost 40% of the genome in some species. They are exceptional in their size (reaching 25 kb) and possess several specific features, including an intron within a polyprotein-coding region, and an extra open reading frame (ORF1) encoding a protein of unknown function located upstream of the gag gene. Although these features make Ogres interesting for further research, identification of additional elements from a broader range of plant taxa has been complicated by the divergence of their sequences, preventing their detection using similarity-based searches. Here we report the results of structure-based computational searches for Ogre elements in available plant genomic sequences, which proved to be more efficient and revealed occurrences of Ogres in three families of dicot plants (Leguminosae, Solanaceae and Salicaceae). In addition, a representative set of 85 elements was retrieved from a model legume species Medicago truncatula. All identified full-length elements were used for comparative analysis, which showed that in spite of only little conservation of their nucleotide sequences, their protein domains were highly conserved, including several regions within ORF1. Further, the elements shared the same functional regions, including a primer binding site complementary to tRNAarg, a conserved motif within a polypurine tract, and a putative intron between the pro and rt/rh coding domains. These findings, together with analysis of their phylogenetic relationship to other retrotransposons based on similarities of rt domains suggest that Ogre elements from different plant taxa have a common origin and thus constitute a distinct group of Ty3/gypsy retrotransposons.