PLUS-STRAND DNA-SYNTHESIS OF THE YEAST RETROTRANSPOSON TY1 IS INITIATED AT 2 SITES, PPT1 NEXT TO THE 3'-LTR AND PPT2 WITHIN THE POL GENE - PPT1 IS SUFFICIENT FOR TY1 TRANSPOSITION

Long terminal repeat elements and retroviruses require primers for ini tiation of minus and plus-strand DNA synthesis by reverse transcriptas e. Here we demonstrate genetically that plus-strand DNA synthesis of t he yeast Ty1 element is initiated at two sites located at the 5' bound ary of the 3' long terminal repeat (PPT1) and near the middle of the p ol gene in the integrase coding sequence (PPT2). A consequence of the presence of two PPTs is that Ty1 plus-strand DNA exists as segments at some time during replication. Three fragments have been identified: t he plus-strand strong-stop DNA initiated at PPT1, a downstream fragmen t initiated at PPT2 and an upstream fragment spanning the 5'-terminal part of Ty1 and a portion of the TyB gene. Characterization of the 3' ends of the plus-strand DNA fragments reveals (1) that the upstream fr agment is elongated beyond PPT2 creating a plus-strand overlap and (2) that the majority of plus-strand strong-stop DNA fragments bear a cop y of the minus-strand primer binding site in agreement with the accept ed model of retroviral genomic RNA reverse transcription. The two poly purine tracts, PPT1 and PPT2, have an identical sequence GGGTGGTA. Mut ations replacing purines by pyrimidines in this sequence significantly diminish or abolish initiation of plus-strand synthesis. Ty1 elements bearing a mutated PPT2 sequence are not defective for transposition w hereas mutations in PPT1 abolish transposition. (C) 1995 Academic Pres s Limited