Heteroduplex formation by recA protein: polarity of strand exchanges.

Abstract
Purified [Escherichia coli] recA protein promotes strand exchanges between linear duplex DNA and homologous circular single-stranded phage .vphi.X174 DNA that carries a short hybridized fragment. The mechanism of this strand exchange reaction was investigated. recA protein initiated strand exchanges by pairing the free end of the duplex fragment with the single-stranded DNA. Strand exchanges were polar and stable heteroduplex molecules were formed by the directional transfer of the (-) strands starting at 3'' termini.