Reverse transcriptase activity and untranslated region sharing of a new RTE-like, non-long terminal repeat retrotransposon from the human blood fluke, Schistosomajaponicum

Abstract

A new RTE-like, non-long terminal repeat retrotransposon, termed SjR2, from the human blood fluke, Schistosoma japonicum, is described. SjR2 is ∼3.9 kb in length and is constituted of a single open reading frame encoding a polyprotein with apurinic/apyrimidinic endonuclease and reverse transcriptase domains. The open reading frame is bounded by 5′- and 3′-terminal untranslated regions and, at its 3′-terminus, SjR2 bears a short (TGAC) 3 repeat. Phylogenetic analyses based on conserved domains of reverse transcriptase or endonuclease revealed that SjR2 belonged to the RTE clade of non-long terminal repeat retrotransposons. Further, SjR2 was homologous, but probably not orthologous, to SR2 from the African blood fluke, Schistosoma mansoni; this RTE-like family of non-long terminal repeat retrotransposons appears to have arisen before the divergence of the extant schistosome species. Hybridisation analyses indicated that ∼10,000 copies of SjR2 were dispersed throughout the S. japonicum chromosomes, accounting for up to 14% of the nuclear genome. Messenger RNAs encoding the reverse transcriptase and endonuclease domains of SjR2 were detected in several developmental stages of the schistosome, indicating that the retrotransposon was actively replicating within the genome of the parasite. Exploration of the coding and non-coding regions of SjR2 revealed two notable characteristics. First, the recombinant reverse transcriptase domain of SjR2 expressed in insect cells primed reverse transcription of SjR2 mRNA in vitro. By contrast, recombinant SjR2-endonuclease did not appear to cleave schistosome or plasmid DNA. Second, the 5′-untranslated region of SjR2 was >80% identical to the 3′-untranslated region of a schistosome heat shock protein-70 gene ( hsp-70) in the antisense orientation, indicating that SjR2-like elements were probably inserted into the non-coding regions of ancestral S. japonicum HSP-70, probably after the species diverged from S. mansoni.