T4 replication: What does 'processivity' really mean?

摘要

DNA polymerases are full of surprises. It is widely accepted that replicative polymerases are highly processive, thanks, in large part, to accessory sub-units 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.