Two distinct triggers for cycling of the lagging strand polymerase at the replication fork

There are two modes of DNA synthesis at a replication fork. The leading str and is synthesized in a continuous fashion in lengths that in Escherichia c oli can be in excess of 2 megabases. On the other hand, the lagging strand is synthesized in relatively short stretches of 2 kilobases. Nevertheless, identical assemblies of the DNA polymerase In: core tethered to the beta sl iding clamp account for both modes of DNA synthesis. Yet the same lagging s trand polymerase accounts for the synthesis of all Okazaki fragments at a r eplication fork, cycling repeatedly every 1 or 2 s from the S'-end of the j ust-completed fragment to the 3'-end of the new primer. Several models have been invoked to account for the rapid cycling of a polymerase complex that can remain bound to the template for upward of 40 min. By using isolated r eplication protein-DNA template complexes, we have tested these models and show here that cycling of the lagging strand polymerase can be triggered by either the action of primase binding to the replisome and synthesizing a p rimer or by collision of the lagging strand polymerase with the 5' end of t he previous Okazaki fragment.