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 chromoso me repair and evolution of gene expression (Teng et al., 1996; McDonald, 19 93). Here, we report the existence of two class of genomic copies of retrot ransposon 1731 with different expression strategies, one of which might be driven by natural selection. The first class uses conventional translation frameshifting known to ensure expression of revere 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 recoginsing rare tRNA by a codon preferred by host genome, whereas the RT ORF is restored by downstream single nuclotide deletion. We suggest that natural selection ha s driven the switching of 1731 expression strategy from retrovirus-like to the fussion-ORF expression. This observation is in accordance with the dete ction in testes of fused Gag-RT polypetide encoded by 1731. The abundance o f RT in testes may serve for normal development of host tissue.