A new MCM modification cycle regulates DNA replication initiation

The hexameric DNA helicase MCM (Mcm2-7) is a central regulatory target in eukaryotic replication. Chromatin-bound MCM is kept inactive during G1 phase and subsequently activated in S phase to initiate replication. During this transition, the only known chemical change on the Mcm2-7 proteins is the gain of multi-site phosphorylation that promotes recruitment of co-factors. As replication initiation is tied intimately to multiple biological cues, additional changes on these proteins can provide a second regulatory point. Here we describe a new MCM modification cycle mediated by SUMO that enables a negative regulation of replication initiation. We show that all MCM subunits undergo sumoylation upon loading at origins in G1 phase prior to MCM phosphorylation. Then bulk MCM sumoylation is lost as MCM phosphorylation rises. The pattern of MCM sumoylation suggests a negative role in replication. Indeed, increasing MCM sumoylation delays genome-wide replication initiation. Mechanistically, this is partly due to enhancing the recruitment of a conserved phosphatase that delays MCM phosphorylation and activation. By revealing a new MCM modification cycle and its role in replication, our findings suggest a new model, in which MCM sumoylation counterbalances kinase-based regulation to ensure accurate control of replication initiation. Two sets of samples at are included: 1) set 1 contains 4 reference samples, collected at 0 min, 15 min, 30 min, 40 min post cdc7-4 release; 2) set 2 contains 4 mutant samples, collected at the same time point as the reference. Whole genome sequencing (Illumina HiSeq) was used to access the copy number change and origin firing in the these samples.