DNA STRAND EXCHANGE IN THE ABSENCE OF HOMOLOGOUS PAIRING

The strand exchange reaction that is widely used for in vitro studies on recombination of DNA molecules is generally presumed to result from a preceding homologous pairing step. With the use of a single-strande d circular DNA and a short homologous linear duplex fragment as substr ates in a model strand exchange reaction, it was found that modest con centrations of polyethylene glycol or salt promote formation of hetero duplex molecules and strand exchange. When the duplex fragment was par tially resected with an exonuclease exposing a short single-stranded s tretch on the ends, the reaction was promoted by the addition of comme rcial bovine serum albumin. The transcription factor TFIIIA promoted s trand exchange when the DNA substrates contained the cognate DNA bindi ng sequence recognized by the protein. These observations suggest that detection of strand exchange in vitro does not necessarily imply a pr eceding homologous pairing step.