S-phase checkpoint proteins Tof1 and Mrc1 form a stable replication-pausing complex

The checkpoint regulatory mechanism has an important role in maintaining the integrity of the genome(1-5). This is particularly important in S phase of the cell cycle, when genomic DNA is most susceptible to various environmental hazards(3,6,7). When chemical agents damage DNA, activation of checkpoint signalling pathways results in a temporary cessation of DNA replication. A replication-pausing complex is believed to be created at the arrested forks to activate further checkpoint cascades, leading to repair of the damaged DNA. Thus, checkpoint factors are thought to act not only to arrest replication but also to maintain a stable replication complex at replication forks(6-9). However, the molecular mechanism coupling checkpoint regulation and replication arrest is unknown. Here we demonstrate that the checkpoint regulatory proteins Tof1 and Mrc1 interact directly with the DNA replication machinery in Saccharomyces cerevisiae. When hydroxyurea blocks chromosomal replication, this assembly forms a stable pausing structure that serves to anchor subsequent DNA repair events.