THE SEARCH FOR THE RIGHT PARTNER - HOMOLOGOUS PAIRING AND DNA STRAND EXCHANGE PROTEINS IN EUKARYOTES

Finding the right partner is a central problem in homologous recombina tion. Common to all models for general recombination is a homologous p airing and DNA strand exchange step. In prokaryotes this process has m ainly been studied with the RecA protein of Escherichia coli. Two appr oaches have been used to find homologous pairing and DNA strand exchan ge proteins in eukaryotes. A biochemical approach has resulted in nume rous proteins from various organisms. Almost all of these proteins are biochemically fundamentally different from RecA. The in vivo role of these proteins is largely not understood. A molecular-genetical approa ch has identified structural homologs to the E. coli RecA protein in t he yeast Saccharomyces cerevisiae and subsequently in other organisms including other fungi, mammals, birds, and plants. The biochemistry of the eukaryotic RecA homologs is largely unsolved. For the fungal RecA homologs (S. cerevisiae RAD51, RAD55, RAD57, DMC1; Schizosaccharomyce s pombe rad51; Neurospora crassa mei3) a role in homologous recombinat ion and recombinational repair is evident. Besides recombination, homo logous pairing proteins might be involved in other cellular processes like chromosome pairing or gene inactivation.