The retrotransposon Osvaldo from Drosophila buzzatii displays all structural features of a functional retrovirus

The Osvaldo retrotransposon has shown a high transposition rate in some str ains of Drosophila buzzaiti and in hybrids between D. buzzatii and its sibl ing D. koepferae. In order to understand the molecular basis of this phenom enon, we developed a procedure to clone a recently transposed copy with the aim of characterizing an active, full-length Osvaldo element. The complete nucleotide sequence of Osvaldo, obtained from a recent insertion site, was determined. Osvaldo is 9,045 bp long and is composed of a central coding r egion flanked by identical long terminal repeats (LTRs) of 1,196 bp each. S equences homologous to the polypurine tract and tRNA-primer-binding site of retroviruses are located adjacent to the 3' and 5' LTRs, respectively. The internal region of Osvaldo contains three long open reading frames (ORFs 1 , 2, and 3), comparable in size and location to gag, pal, and env retrovira l genes. The conceptual translation of Osvaldo ORF1 exhibits sequence homol ogy to HIV1 and SIV capsid (p24) and nucleocapsid (p7) mature proteins. ORF 2 encodes the putative protease (PR), reverse transcriptase/ribonuclease H (RT/RH), integrase (IN), and a significant portion of the surface envelope (ENV) protein that is interrupted by a putative intron. A third ORF encodes the remaining part of the ENV protein. The predicted 62-kDa ENV protein sh ares several general features with membrane glycoproteins, including a pote ntial signal peptide, a transmembrane domain near the C-terminus that could function as a membrane anchor, four consensus N-linked glycosylation motif s, and, finally, a potential protease cleavage site. The phylogenetic relat ionships of Osvaldo are explored, and they suggest that Osvaldo may constit ute a new family of retroviruses in insects, distantly related to the previ ously described group of gypsy retroviruses.