Cross-kingdom commonality of a novel insertion signature of RTE-related short retroposons

Abstract

In multicellular organisms, such as vertebrates andflowering plants, horizontal transfer (HT) of genetic information is thought to be a rare event.However, recent findings unveiled unexpectedly frequentHT of RTE-clade LINEs. To elucidate the molecular footprints of the genomic integration machinery of RTE-related retroposons, the sequence patterns surrounding the insertion sites of plant Au-like SINE families were analyzed in the genomes of a wide variety of flowering plants. A novel and remarkable finding regarding target site duplications (TSDs) for SINEswas they startwith thymine approximately one helical pitch (ten nucleotides) downstreamof a thymine stretch. This TSD pattern was found in RTE-clade LINEs, which share the 3′-end sequence of these SINEs, in the genome of leguminous plants. These results demonstrably show that Au-like SINEs were mobilized by the enzymatic machinery of RTEclade LINEs. Further, we discovered the same TSD pattern in animal SINEs from lizard and mammals, in which the RTE-clade LINEs sharing the 3′-end sequence with these animal SINEs showed a distinct TSD pattern. Moreover, a significant correlation was observed between the first nucleotide of TSDs and microsatellite-like sequences found at the 3′-ends of SINEs and LINEs.We propose that RTE-encoded protein could preferentially bind to a DNA region that contains a thymine stretch to cleave a phosphodiester bond downstream of the stretch. Further, determination of cleavage sites and/or efficiency of primer sites for reverse transcription may depend on microsatellite-like repeats in the RNA template. Such a unique mechanism may have enabled retroposons to successfully expand in frontier genomes after HT.