A rare family of gtwin retrotransposon carrying a mutation in the tRNA-primer binding site is amplified in G-32 Drosophila melanogaster strain

Abstract

The LTRretrotransposon gtwin of Drosophila mel� anogaster was first cloned from the laboratory strain G�32, which is characterized by a strong amplification of copies of this transposable element (1). In addition, strain G�32 has a number of other unique features, such as the presence in the majority of studied species of a complex extended inversion in the third chromo� some, within which gtwin insertions are accumulated (2), as well as an increased frequency of occurrence of singlestranded DNA breaks, which cause recombina� tion processes and gene conversion (3). Currently, gtwin transpositions in strain G�32 are not observed; however, extrachromosomal circular copies of this ele� ment containing two polymorphic sites in the binding sequence of the tRNA primer for the synthesis of the minus strand of DNA were identified in the genome. Among the analyzed clones containing relevant 5'� untranslated region of circular gtwin molecules, the group with a mutant binding site for the tRNA primer represented by noncomplementary nucleotides at positions 497 and 503 was found to be the most numerous. In addition, four fulllength gtwin copies cloned from the genomic library of strain G�32 also belonged to the subfamily with the "incorrect" bind� ing site for the tRNA primer (4). In this work we performed molecular analysis of different copies of LTR retrotransposon gtwin, which were cloned from strain G�32, and showed that, although its genome contains variants with either nor� mal or mutant binding site for the tRNA primer for the synthesis of the minus strand of DNA, it is the mutant subfamily that was amplified. In addition, we searched different gtwin variants in 14 laboratory strains of D. melanogaster and in D. melanogaster Schneider 2 cells. We demonstrated the existence of two subfam� ilies of the