Homologous pairing and strand exchange promoted by the Escherichia coli RecT protein.

RecT protein of Escherichia coli promotes the formation of joint molecules between homologous linear double-stranded M13mp19 replicative-form bacteriophage DNA and circular single-stranded M13mp19 DNA in the presence of exonuclease VIII, the recE gene product. The joint molecules were formed by a mechanism involving the pairing of the complementary strand of the linear double-stranded DNA substrate with the circular single-stranded DNA substrate coupled with the displacement of the noncomplementary strand. When the homologous linear double-stranded DNA substrate had homologous 3' or 5' single-stranded tails, then RecT promoted homologous pairing and strand exchange in the absence of exonuclease VIII. Histone H1 could substitute for RecT protein; however, joint molecules formed in the presence of histone H1 did not undergo strand exchange. These results indicate that under the reaction conditions used, the observed strand exchange reaction is promoted by RecT and is not the result of spontaneous branch migration. These results are consistent with the observation that expression of RecE (exonuclease VIII) and RecT substitutes for RecA in some recombination reactions in E. coli.