Crystal structure of the Red beta C-terminal domain in complex with lambda Exonuclease reveals an unexpected homology with lambda Orf and an interaction with Escherichia coli single stranded DNA binding protein.

Bacteriophage λ encodes a DNA recombination system that includes a 5′-3′ exonuclease (λ Exo) and a single strand annealing protein (Redβ). The two proteins form a complex that is thought to mediate loading of Redβ directly onto the single-stranded 3′-overhang generated by λ Exo. Here, we present a 2.3 A crystal structure of the λ Exo trimer bound to three copies of the Redβ C-terminal domain (CTD). Mutation of residues at the hydrophobic core of the interface disrupts complex formation in vitro and impairs recombination in vivo. The Redβ CTD forms a three-helix bundle with unexpected structural homology to phage λ Orf, a protein that binds to E. coli single-stranded DNA binding protein (SSB) to function as a recombination mediator. Based on this relationship, we found that Redβ binds to full-length SSB, and to a peptide corresponding to its nine C-terminal residues, in an interaction that requires the CTD. These results suggest a dual role of the CTD, first in binding to λ Exo to facilitate loading of Redβ directly onto the initial single-stranded DNA (ssDNA) at a 3′-overhang, and second in binding to SSB to facilitate annealing of the overhang to SSB-coated ssDNA at the replication fork.