Constitutive ATR Signaling Enforces Domain-Wide Origin Synchrony
Eukaryotic DNA replication relies on organized spatial-temporal domains, yet how neighboring origins are coordinated at the single-molecule level remains unresolved. Combining single-molecule DNA combing with super-resolution microscopy, we demonstrate that during unperturbed early S-phase, adjacent replication origins across diverse vertebrate species fire synchronously as a coordinated unit. This domain-wide temporal harmony is continuously enforced by a baseline, constitutive ATR CHK1 signaling axis that restrains local CDK2 and CDC7 kinase activities. Strikingly, replication stress dismantles this coordination, shifting origin activation toward an asynchronous, replicon-autonomous mode. Mechanistically, stress-induced phosphorylation of the cohesin subunit SMC1A promotes an SMC1A ATR complex that sequesters ATR at stalled replication forks. This localized sequestration creates a regional ATR loss-of-function phenotype, relieving CDK2/CDC7 suppression and driving premature, uncoordinated origin firing. Super-resolution imaging shows that ATR sequestration structurally reorganizes replication foci, converting coordinated clustered foci into fragmented, lower-density clusters. Together, our findings uncover a baseline role for constitutive ATR signaling in enforcing domain synchrony and reveal a molecular switch that dynamically decouples origin activation during genomic stress to safeguard chromosome duplication.