The complete Tirant transposable element in Drososphila melanogaster showsa structural relationship with retrovirus-like retrotransposons

We have determined the structure and organization of Tirant, a retrotranspo son of Drosophila melanogaster reported in literature to be responsible for four independent mutations. Tirant is a long terminal repeat (LTR) retrotr ansposon 8527 bp long. It possesses three open reading frames (ORF) encodin g Gag, Pol and Env proteins with a strong similarity with ZAM, a recently i dentified member of the gypsy class of retrovirus-like mobile elements. Mol ecular analysis of the Tirant genomic copies present in four D. melanogaste r strains revealed that most of them are defective, non-autonomous elements that differ in the position and extension of the conserved internal portio n. Defective elements lacking the Gag ORF but retaining the Env ORF are abu ndant in heterochromatin. Four discrete Tirant transcripts are observed dur ing embryogenesis in the strain Oregon-R, the smaller of which, 1.8 kb in s ize, originates from the splicing of a primary transcript and leads to a su bgenomic RNA coding for the Env product. (C) 2000 Elsevier Science B.V. All rights reserved.