BRANCH MIGRATION DURING RAD51-PROMOTED STRAND EXCHANGE PROCEEDS IN EITHER DIRECTION

The Saccharomyces cerevisiae Rad51 protein is important for genetic re combination and repair of DNA double-strand breaks in vivo and can pro mote strand exchange between linear double-stranded DNA and circular s ingle-stranded DNA in vitro. However, unlike Escherichia coil RecA, Ra d51 requires an overhanging complementary 3' or 5' end to initiate str and exchange; given that fact, we previously surmised that the fully e xchanged molecules resulted from branch migration in either direction depending on which type of end initiated the joint molecule, Our prese nt experiments confirm that branch migration proceeds in either direct ion, the polarity depending on whether a 3' or 5' end initiates the jo int molecules. Furthermore, heteroduplex DNA is formed rapidly, first at the overhanging end of the linear double-stranded DNA's complementa ry strand and then more slowly by progressive lengthening of the heter oduplex region until strand exchange is complete. Although joint molec ule formation occurs equally efficiently,when initiated with a 3' or 5 ' overhanging end, branch migration proceeds more rapidly when it is i nitiated by an overhanging 3' end, i.e., in the 5' to 3' direction wit h respect to the single-stranded DNA.