Torsional stress generated by RecA protein during DNA strand exchange separates strands of a heterologous insert.

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Abstract
Previous studies have shown that the helical RecA nucleoprotein filament formed on a circular single strand of DNA causes the progressive, directional transfer of a complementary strand from naked linear duplex DNA to the nucleoprotein filament, even when the duplex contains a sizable heterologous insertion. Since RecA protein lacks demonstrable helicase activity, the mechanism by which it pushes strand exchange through long heterologous inserts has been a quandary. In the present study, a linear duplex substrate with an insertion of 110 base pairs in its middle yielded the expected products, whereas much less of the heteroduplex product was seen when the insertion was located at either end of the duplex substrate or 160 base pairs from the far end of the duplex substrate. In an ongoing reaction of the substrate with an insertion in its middle, P1 nuclease cleaved intermediates from the point of the insertion to various distal sites. Acting on a duplex substrate that contained a single nick located in the complementary strand just beyond the insertion, RecA protein formed joint molecules but failed to complete strand exchange. These data show that negative torsional stress is generated by distant homologous interactions that occur beyond the heterologous insertion and that such stress is essential for unwinding a heterologous insertion that otherwise halts strand exchange.