?Checkpoint inhibition of origin firing prevents ?inappropriate replication outside of S-phase

Across eukaryotes, checkpoints maintain the order of cell cycle events in the face of DNA damage or incomplete replication. Although a wide array of DNA lesions activates the checkpoint kinases, whether and how this response differs in different phases of the cell cycle remains poorly understood. The S-phase checkpoint for example results in the slowing of replication, which in the budding yeast Saccharomyces cerevisiae is caused by Rad53 kinase-dependent inhibition of the initiation factors Sld3 and Dbf4. Despite this, we show here that Rad53 phosphorylates both of these substrates throughout the cell cycle at the same sites as in S-phase, suggesting roles for this pathway beyond S-phase. Indeed we show that Rad53-dependent inhibition of Sld3 and Dbf4 limits re-replication in G2/M phase, preventing inappropriate gene amplification events. In addition we show that inhibition of Sld3 and Dbf4 after DNA damage in G1 phase prevents premature replication initiation at all origins at the G1/S transition. This study redefines the scope and specificity of the 'S-phase checkpoint' with implications for understanding the roles of this checkpoint in the majority of cancers that lack proper cell cycle controls. Overall design: Examination of replication progression by high-throughput sequencing of DNA in two strains (swi4-t and swi4-t/sld3-A/dbf4-A) after DNA damage in G1 phase.