Structure of the gene 2.5 protein, a single-stranded DNA binding protein encoded by bacteriophage T7

The gene 2.5 protein (gp2.5) of bacteriophage T7 is a single-stranded DNA ( ssDNA) binding protein that has essential roles in DNA replication and reco mbination. In addition to binding DNA, gp2.5 physically interacts with T7 D NA polymerase and T7 primase-helicase during replication to coordinate even ts at the replication fork. We have determined a 1.9-Angstrom crystal struc ture of gp2.5 and show that it has a conserved OB-fold (oligosaccharide/oli gonucleoticle binding fold) that is well adapted for interactions with ssDN A. Superposition of the OB-folds of gp2.5 and other ssDNA binding proteins reveals a conserved patch of aromatic residues that stack against the bases of ssDNA in the other crystal structures, suggesting that gp2.5 binds to s sDNA in a similar manner. An acidic C-terminal extension of the gp2.5 prote in, which is required for dimer formation and for interactions with the T7 DNA polymerase and the primase-helicase, appears to be flexible and may act as a switch that modulates the DNA binding affinity of gp2.5.