QUANTITATIVE RECA PROTEIN-BINDING TO THE HYBRID DUPLEX PRODUCT OF DNASTRAND EXCHANGE

Following a DNA strand exchange reaction, RecA protein remains bound t o the hybrid DNA product. DNA strand exchange reactions were carried o ut under optimal conditions in the presence of both RecA protein and S SB protein. As monitored by a sensitive DNA underwinding assay, all of the RecA protein present in the RecA nucleoprotein filament that init iates the strand exchange reaction can be accounted for on the hybrid DNA. As shown elsewhere, the SSB is bound to the displaced single DNA strand. Previous studies showed that RecA protein will dissociate from dsDNA when ADP levels build up, or transfer from dsDNA to ssDNA when the latter is not bound by SSB. The present work (done with ATP regene ration and SSB) shows that efficient strand exchange occurs in the abs ence of a net dissociation or transfer of RecA monomers from the filam ent. Such a dissociation or transfer is therefore not a mechanistic re quirement for DNA strand exchange. The results provide evidence agains t some models proposed for the DNA strand exchange mechanism.