RNA FACILITATES RECA-MEDIATED DNA PAIRING AND STRAND TRANSFER BETWEENMOLECULES BEARING LIMITED REGIONS OF HOMOLOGY

The RecA protein of Escherichia coli catalyzes homologous pairing and strand exchange between a wide range of molecules showing nucleotide s equence complementarity, including a linear duplex and a single-strand ed DNA molecule. We demonstrate that RecA can promote formation of joi nt molecules when the duplex contains an RNA/DNA hairpin and a single- stranded circle serves as the pairing partner, A chimeric RNA/DNA hair pin can be used to form stable joint molecules with as little as 15 ba ses of shared homology as long as the RNA stretch contains complementa rity to the circle. The joint molecule bears some resemblance to a tri ple helical structure composed of RNA residues surrounded by two DNA s trands which are in a parallel orientation. Evidence is presented that supports the notion that short stretches of RNA can be used in homolo gous pairing reactions at lengths below that required for DNA-DNA hete roduplex formation.