Molecular evolution of Orf1 gene in plant LTR-retrotransposons

LTR-Retrotransposons are the major DNA components in plant genomes. They usually contain gag and pol, two genes necessary for transpositinal process. Our previous study on soybean genome annotation identified a SARE LTR-Retrotransposon family, which carries the third gene, Orf1. Using a bioinformatics approach, we here reported that 7 out of 33 sequenced genomes have some LTR-Retrotransposons with an extra Orf1 gene/gene fragment (approximately 1-2 kb) in the region between 5' LTR and gag gene, including Eucalyptus grandis, Populus trichocarpa, Gossypium raimondii, Glycine max, Lotus japonica, Linum usitatissimum, and Medicago truncatula. The majority of these elements were inserted into the genomes they reside within the last 3 million years, but their structures, frequencies, intensity, and activity in different host genomes are quite different. Phylogenetic analysis indicated that these unusual elements were clustered in a eudicot branch, suggesting that they may be generated in the evolution of some eudicot species. The relative conservation, transcriptional activity, and the presence of multiple potential conserved motifs suggest that Orf1 gene may still be functional.