DIFFERENT ORGANIZATION OF THE TRANSFER RNA-GENE-ASSOCIATED REPETITIVEELEMENT, DRE, IN NC4-DERIVED STRAINS AND IN OTHER WILD-TYPE DICTYOSTELIUM-DISCOIDEUM STRAINS

The retrotransposon DRE (Dictyostelium repetitive element was discover ed in the course of an extensive study concerning the genomic organiza tion of tRNA genes in the NC4-derived strains AX2 and AX3 of the cellu lar slime mold Dictyostelium discoideum. As a striking feature, DRE wa s found exclusively in a constant orientation and at a constant distan ce upstream from different tRNA genes. About 150-200 DRE with intact 5 '-terminal-repeat structures are present in NC4-derived strains. These strains were termed high-copy DRE strains (HCD strains) as opposed to low-copy DRE strains (LCD strains) such as the wild-type D. discoideu m isolates DD61, WS380B, OHIO and V12. LCD strains contain only 3-15 D RE with intact 5'-terminal-repeat-structures. However, in addition to these few intact elements, many 5'-truncated DRE elements are present in LCD strains. In HCD strains, most DRE show typical structural chara cteristics of retrotransposons containing terminal repeats at both end s, which seems to be one prerequisite for active transposition. In LCD strains, however, most DRE elements are 5'-truncated, which is a comm on feature of eukaryotic LINE elements. Despite their truncated 5'-end s, DRE in LCD strains retain unique integration specificities, i.e. th ey are always found position-specifically and orientation-specifically integrated in front of tRNA genes, flanked by a 12-16-bp target-site duplication.