The mechanism of vertebrate DNA replication termination

Harvard Medical School, Boston MA

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Abstract

? DESCRIPTION (provided by applicant): Vertebrate chromosomes are replicated from thousands of origins of replication. At each origin, two replication forks are established that travel in opposite directions, copying DNA as they go. When converging forks meet, replication terminates. Termination involves local completion of DNA synthesis, decatenation of daughter molecules, and replisome disassembly. Failure to properly execute these steps leads to genome instability, a hallmark of most cancers. Despite its importance, the mechanism of termination is poorly understood, largely because the timing and location of termination events is not well-defined. As a result, termination is difficult to study using conventional approaches such as chromatin immunoprecipitation. To overcome these challenges, we developed an approach to induce localized and synchronous termination events in Xenopus egg extracts. A plasmid containing an array of lac operator (lacO) sites is incubated with lac repressor (LacR) and added to egg extract. DNA replication initiates somewhere on the plasmid and two forks converge on the outer edges of the LacR array, where they stall. Upon addition of IPTG, LacR dissociates, forks immediately resume elongation, and they converge and terminate synchronously in a small region within the lacO array. Using this approach, we have undertaken the most detailed mechanistic dissection of termination to date, leading to a fundamentally new model of this process. We show that there is no detectable slowing of DNA synthesis as forks converge and that dissociation of the two replicative CMG helicases occurs after leading strands are ligated to downstream Okazaki fragments of the converging fork. This observation implies that when CMGs reach the downstream Okazaki fragment, they move from ssDNA onto dsDNA, from where they are unloaded. We also found that CMG unloading involves ubiquitylation of its MCM7 subunit and the action of the p97 ATPase. In this proposal, we will use our new cell-free system to further elucidate the mechanism of replication termination. We will test our novel hypothesis that the interaction of CMG with dsDNA triggers CMG unloading (Aim 1). We will identify the E3 ubiquitin ligase that ubiquitylates MCM7 to promote CMG unloading, as well as any adaptor proteins that cooperate with p97 to extract CMG from DNA (Aim 2). We will also determine how the E3 ubiquitin ligase and p97 recognize terminated CMGs. Finally, we will address how failure to unload CMG affects DNA replication and DNA repair (Aim 3). Our experiments will lead to the first comprehensive molecular description of eukaryotic replication termination and elucidate how the proper regulation of this process suppresses genome instability.