Low dependency of retrotransposition on the ORF1 protein of the zebrafish LINE, ZfL2-1

The zebrafish long interspersed element (LINE), ZfL2-1, which belongs to the L2 clade, contains two open reading frames, ORF1 and ORF2. ORF1 encodes a protein containing a coiled-coil motif and an esterase domain, whereas ORF2 encodes a protein containing an endonuclease and a reverse transcriptase domain. To elucidate the functional significance of ORF1 in retrotransposition, we constructed many variants of ZfL2-1 and examined their retrotransposition ability. We concluded: 1) the ORF1 protein is not essential for ZfL2-1 retrotransposition in cultured cells; 2) the translation of ORF1 is required for the translation of ORF2; and 3) ORF2 translation probably occurs via suppression of the ORF1 stop codon, the efficiency of which is influenced by the context of the sequence juxtaposed to the 3′ side of the stop codon. These results offer a new perspective on the evolution of the L2 clade LINEs.

► The ORF1 protein has no apparent essential function in retrotransposition of ZfL2-1. ► An upstream coding sequence, however, is required for retrotransposition of ZfL2-1. ► We propose a model for the translation of ORF2 through the ORF1 in ZfL2-1.