A unique loop in T7 DNA polymerase mediates the binding of helicase-primase, DNA binding protein, and processivity factor

Bacteriophage T7 DNA polymerase (gene 5 protein, gp5) interacts with its processivity factor, Escherichia coli thioredoxin, via a unique loop at the tip of the thumb subdomain. We find that this thioredoxin-binding domain is also the site of interaction of the phage-encoded helicase/primase (gp4) and ssDNA binding protein (gp2.5). Thioredoxin itself interacts only weakly with gp4 and gp2.5 but drastically enhances their binding to gp5. The acidic C termini of gp4 and gp2.5 are critical for this interaction in the absence of DNA. However, the C-terminal tail of gp4 is not required for binding to gp5 when the latter is bound to a primer/template. We propose that the thioredoxin-binding domain is a molecular switch that regulates the interaction of T7 DNA polymerase with other proteins of the replisome.