τ Protects β in the Leading-strand Polymerase Complex at the Replication Fork (∗)

Replication forks formed in the absence of the τ subunit of the DNA polymerase III holoenzyme produce shorter leading and lagging strands than when τ is present. We show that one reason for this is that in the absence of τ, but in the presence of the γ-complex, leading-strand synthesis is no longer highly processive. In the absence of τ, the size of the leading strand becomes proportional to the concentration of β and inversely proportional to the concentration of the γ-complex. In addition, the β in the leading-strand complex is no longer resistant to challenge by either anti-β antibodies or poly(dA):oligo(dT). Thus, τ is required to cement a processive leading-strand complex, presumably by preventing removal of β catalyzed by the γ-complex.