Boudicca , a Retrovirus-Like Long Terminal Repeat Retrotransposon from the Genome of the Human Blood Fluke Schistosoma mansoni

Abstract

The genome of Schistosoma mansoni contains a proviral form of a retrovirus-like long terminal repeat (LTR) retrotransposon, designated Boudicca . Sequence and structural characterization of the new mobile genetic element, which was found in bacterial artificial chromosomes prepared from S. mansoni genomic DNA, revealed the presence of three putative open reading frames (ORFs) bounded by direct LTRs of 328 bp in length. ORF1 encoded a retrovirus-like major homology region and a Cys/His box motif, also present in Gag polyproteins of related retrotransposons and retroviruses. ORF2 encoded enzymatic domains and motifs characteristic of a retrovirus-like polyprotein, including aspartic protease, reverse transcriptase, RNase H, and integrase, in that order, a domain order similar to that of the gypsy / Ty3 retrotransposons. An additional ORF at the 3′ end of the retrotransposon may encode an envelope protein. Phylogenetic comparison based on the reverse transcriptase domain of ORF2 confirmed that Boudicca was a gypsy -like retrotransposon and showed that it was most closely related to CsRn1 from the Oriental liver fluke Clonorchis sinensis and to kabuki from Bombyx mori . Bioinformatics approaches together with Southern hybridization analysis of genomic DNA of S. mansoni and the screening of a bacterial artificial chromosome library representing ≈8-fold coverage of the S. mansoni genome revealed that numerous copies of Boudicca were interspersed throughout the schistosome genome. By reverse transcription-PCR, mRNA transcripts were detected in the sporocyst, cercaria, and adult developmental stages of S. mansoni , indicating that Boudicca is actively transcribed in this trematode.