Detection of extrachromosomal linear DNAs of LTR retrotransposons by ALE-seq

Retrotransposons have played an important role in the evolution of host genomes. Their impact on host chromosomes is mainly deduced from the composition of DNA sequences, which have been fixed over evolutionary time. These studies provide important “snapshots” reflecting historical activities of transposons but do not predict current transposition potential. We previously reported Sequence-Independent Retrotransposon Trapping (SIRT) as a methodology that, by identification of extrachromosomal linear DNA (eclDNA), revealed the presence of active LTR retrotransposons in Arabidopsis. Unfortunately, SIRT cannot be applied to large and transposon-rich genomes of crop plants. We have since developed an alternative approach named ALE-seq (amplification of LTR of eclDNAs followed by sequencing). ALE-seq reveals sequences of 5' LTRs of eclDNAs after two-step amplification: in vitro transcription and subsequent reverse transcription. Using ALE-seq in rice, we detected eclDNAs for a novel Copia family LTR retrotransposon, Go-on, which is activated by heat stress. Furthermore, ALE-seq applied to tomato fruits identified a developmentally regulated Gypsy family of retrotransposons.