Structure of DRE, a Retrotransposable Element Which Integrates with Position Specificity Upstream of Dictyostelium discoideum tRNA Genes

Abstract

Different Dictyostelium discoideum strains contain between 2 and 200 copies of a retrotransposable element termed DRE (Dictyostelium repetitive element). From the analysis of more than 50 elements, it can be concluded that DRE elements always occur 50 ± 3 nucleotides upstream of tRNA genes. All analyzed clones contain DRE in a constant orientation relative to the tRNA gene, implying orientation specificity as well as position specificity. DRE contains two open reading frames which are flanked by nonidentical terminal repeats. Long terminal repeats (LTRs) are composed of three distinct modules, called A, B, and C. The tRNA gene-proximal LTR is characterized by one or multiple A modules followed by a single B module (A_nB). With respect to the distal LTR, two different subforms of DRE have been isolated. The majority of isolated clones contains a distal LTR composed of a B module followed by a C module (BC), whereas the distal LTR of the other subform contains a consecutive array of a B module, a C module, a slightly altered A module, another B module, and another C module (BC · ABC). Full-length as well as smaller transcripts from DRE elements have been detected, but in comparison with the high copy number in D. discoideum strains derived from the wild-type strain NC4, transcription is rather poor.