THE SEP1 STRAND EXCHANGE PROTEIN FROM SACCHAROMYCES-CEREVISIAE PROMOTES A PARANEMIC JOINT BETWEEN HOMOLOGOUS DNA-MOLECULES

Strand exchange protein 1 (Sep1) from the yeast Saccharomyces cerevisi ae promotes the transfer of one strand of a linear duplex DNA to a hom ologous single-stranded DNA circle. Using a nitrocellulose filter bind ing assay and electron microscopy, we find that Sep1 promotes the pair ing of homologous DNA molecules via a paranemic joint. In this joint t here is no net interwinding of the parental DNA molecules, as in the s tandard plectonemic double helix. The paranemic joints form with as li ttle as 41 bp of homology between the parental DNA molecules. The subs trates used were a circular molecule (either single-stranded DNA or du plex supercoiled DNA) and a linear duplex with heterologous regions at both ends to bar duplex plectonemic intertwining. We excluded the pos sibility that the exonuclease activity of Sep1 exposes complementary s ingle-stranded regions that constitute the joint. The paranemic joint is the key intermediate in the search for homologous DNA by the RecA p rotein of Escherichia coli. Our results imply that the search process in a eukaryote such as yeast can be mechanistically similar.