Retrotransposon 1731 in Drosophila melanogaster changes retrovirus-like expression strategy in host genome

Earlier related to parasitic elements, retrotransposons of eukaryotes have been demonstrated to participate in general cell processes such as chromosome repair and evolution of gene expression (Teng et al., 1996; McDonald, 1993). Here, we report the existence of two classes of genomic copies of retrotransposon 1731 with different expression strategies, one of which might be driven by natural selection. The first class uses conventional translational frameshifting known to ensure expression of reverse transcriptase (RT) open reading frame (ORF), depending on the efficiency of frameshifting. The bulk of genomic copies are related to the second class where the frameshift is prevented as a result of the substitution of a rare codon recognising rare tRNA by a codon preferred by host genome, whereas the RT ORF is restored by downstream single nucleotide deletion. We suggest that natural selection has driven the switching of 1731 expression strategy from retrovirus-like to the fusion-ORF expression. This observation is in accordance with the detection in testes of fused Gag-RT polypeptide encoded by 1731. The abundance of RT in testes may serve for normal development of host tissue.