CORRELATED TEMPLATE-SWITCHING EVENTS DURING MINUS-STRAND DNA-SYNTHESIS - A MECHANISM FOR HIGH NEGATIVE INTERFERENCE DURING RETROVIRAL RECOMBINATION

Two models for the mechanism of retroviral recombination hare been pro posed: forced copy choice (minus-strand recombination) and strand disp lacement-assimilntion (plus-strand recombination), Each minus-strand r ecombination event results in one template switch, whereas each plus-s trand recombination event results in two template switches, Recombinan t proviruses with one and more than one template switches were previou sly observed, Recombinants with one template switch were generated by minus-strand recombination, while recombinants containing more than on e template switch may have been generated by plus-strand recombination or by correlated minus-strand recombination, We recently observed tha t retroviral recombination exhibits high negative interference whereby the frequency of recombinants containing multiple template su itching events is higher than expected, To delineate the mechanism that gener ates recombinants with more than one template switch, we devised a sys tem that permits only minus-strand recombination. Two highly homologou s vectors, WH204 and WH221, containing eight different restriction sit e markers were used. The primer binding site (PBS) of WH221 was delete d; although reverse transcription cannot initiate from WH221 RNA, it c an serve as a template for DNA synthesis in heterozygotic virions. Aft er one round of retroviral replication, the structures of the recombin ant proviruses were examined, Recombinants containing two, three, four , and Ave template switches were observed at 1.4-, 10-, 65-, and 50-fo ld-higher frequencies, respectively, than expected, This indicates tha t minus-strand recombination events are correlated and can generate pr oviruses with multiple template switches efficiently, The frequencies of recombinants containing multiple template switches were similar to those observed in the previous system, which allowed both minus- and p lus-strand recombination, Thus, the previously reported high negative interference during retroviral recombination can he caused by correlat ed template switches during minus strand DNA synthesis. In addition, a ll examined recombinants contained an intact PBS, indicating that most of the plus-strand DNA transfer occurs after completion of the strong -stop DNA.