Rings and filaments of beta protein from bacteriophage lambda suggest a superfamily of recombination proteins

The beta protein of bacteriophage A acts in homologous genetic recombinatio n by catalyzing the annealing of complementary single-stranded DNA produced by the A exonuclease. It has been shown that the beta protein binds to the products of the annealing reaction more tightly than to the initial substr ates. We find that beta protein exists in three structural states. In the a bsence of DNA, beta protein forms inactive rings with approximate to 12 sub units, The active form of the beta protein in the presence of oligonucleoti des or single-stranded DNA is a ring, composed of approximate to 15-18 subu nits, The double-stranded products of the annealing reaction catalyzed by t he rings are bound by beta protein in a left-handed helical structure, whic h protects the products from nucleolytic degradation. These observations su ggest structural homology for a family of proteins, including the phage P22 erf, the bacterial RecT, and the eukaryotic Rad52 proteins, all of which a re involved in homologous recombination.