INTERNALLY LOCATED AND OPPOSITELY ORIENTED POLYMERASE-II PROMOTERS DIRECT CONVERGENT TRANSCRIPTION OF A LINE-LIKE RETROELEMENT, THE DICTYOSTELIUM REPETITIVE ELEMENT, FROM DICTYOSTELIUM-DISCOIDEUM

The Dictyostelium discoideum NC4 genome harbors approximately 150 indi vidual copies of a retrotransposable element called the Dictyostelium repetitive element (DRE). This element contains nonidentical terminal repeats (TRs) consisting of conserved building blocks A and B in the l eft TR and B and C in the right TR. Seven different-sized classes of R NA transcripts from these elements were resolved by Northern (RNA) blo t analysis, but their combined abundance was very low. When D. discoid eum cells were grown in the presence of the respiratory chain blocker antimycin A, steady-state concentrations of these RNA species increase d 10- to 20-fold. The D. discoideum genome contains two DRE subtypes, the full-length 5.7-kb DREa and the internally deleted 2.4-kb DREb. Bo th subtypes are transcribed, as confirmed by analysis of cloned cDNA. Primary transcripts from the sense strand originate at nucleotide +1 a nd terminate at two dominant sites, located 21 or 28 nucleotides upstr eam from the 3' end of the elements. The activity of a reasonably stro ng polymerase II promoter in the 5'-terminal A module is slightly upre gulated by the tRNA gene located 50 +/- 4 nucleotides upstream and dra stically reduced by the adjacent B module of the DRE. Transcripts from the opposite DNA strand (complementary-sense transcripts) were also d etected, directed by an internally located polymerase II promoter resi ding within the C module. This latter transcription was initiated at m ultiple sites within the oligo(dA(12)) stretch which terminates DREs.