Schizosaccharomyces pombe fatty acid synthase mediates DNA strand exchange in vitro

During purification of a strand exchange activity from Schizosaccharomyces pombe using the three-strand reaction of double-stranded linear and circular single-stranded DNA, we identified p190/210 as an activity that stimulated the strand exchange activity of p140exo2 by about 10-fold. The accompanying report (Käslin, E., and Heyer, W.-D. (1994) J. Biol. Chem. 269, 0000-0000) described the purification and characterization of p140exo2, likely to be the S. pombe homolog of the Saccharomyces cerevisiae strand exchange protein p175SEP1. Here, we report the purification of p190/210 from S. pombe cells and its identification as fatty acid synthase (FAS). S. pombe FAS (p190/210) binds to single-stranded and double-stranded DNA, leading to condensation of DNA into large aggregates. In addition, it is capable of renaturing complementary single-stranded DNA. Besides stimulating the strand exchange activity of p140exo2, FAS (p190/210) itself exhibits strand exchange activity provided the double-stranded substrate has single-stranded tails. We propose a probable mechanism for the action of FAS (p190/210) during DNA strand exchange in vitro. Since FAS (p190/210) is highly unlikely to have a role in homologous recombination in vivo, we discuss the implications of our data on the interpretation of other homologous pairing and strand exchange proteins purified from eukaryotes using this or similar assays.