Rings and filaments of β protein from bacteriophage λ suggest a superfamily of recombination proteins

The β protein of bacteriophage λ acts in homologous genetic recombination by catalyzing the annealing of complementary single-stranded DNA produced by the λ exonuclease. It has been shown that the β protein binds to the products of the annealing reaction more tightly than to the initial substrates. We find that β protein exists in three structural states. In the absence of DNA, β protein forms inactive rings with ≈12 subunits. The active form of the β protein in the presence of oligonucleotides or single-stranded DNA is a ring, composed of ≈15–18 subunits. The double-stranded products of the annealing reaction catalyzed by the rings are bound by β protein in a left-handed helical structure, which protects the products from nucleolytic degradation. These observations suggest structural homology for a family of proteins, including the phage P22 erf, the bacterial RecT, and the eukaryotic Rad52 proteins, all of which are involved in homologous recombination.