T4 replication: What does “processivity” really mean?
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520
The exchange of polymerase subunits in the T4 replisome is mediated by the gp45 sliding clamp.
DNA polymerases are full of surprises. It is widely accepted that replicative polymerases are highly processive, thanks, in large part, to accessory subunits that act as sliding clamps and tether the polymerase to its DNA template. The bacteriophage T4 replication system is typical in that processivity measurements indicate that a substantial fraction of replication complexes should remain associated with DNA for the ≈15 min required to copy an entire genome length (1). However, an elegant study by Yang et al. (2) in this issue of PNAS challenges our notion of processivity by showing that, within the highly processive T4 replisome, individual DNA polymerase molecules are exchanging rapidly, maybe 90 times per genome length under the conditions prevailing in vivo. Thus, if the replisome is visualized as a molecular factory, the workers are making frequent shift changes.
Processivity Measurements
The bacteriophage T4 replisome, on which these studies were carried out, serves as a simple model incorporating the essential features of multisubunit replication systems. The components of the T4 system are illustrated in cartoon form in Fig. 1. The high overall processivity of the replisome does not necessarily mean that all subunits are equally processive, and two approaches have been informative in measuring the life-times of individual components within an actively synthesizing replisome. The simplest is to determine the sensitivity of DNA synthesis to dilution of individual components. This approach has demonstrated that the helicase (gp41) is very stably associated with the replication fork (1). By contrast, lagging strand synthesis was sensitive to dilution of the clamp (gp45), clamp loader (gp44/62 complex), and primase (gp61) (5, 6), reflecting the remodeling of the lagging strand machinery that must take place as each new Okazaki fragment is started. Significantly, dilution of the polymerase (gp43) did not affect lagging strand synthesis, implying that …
