Plant-specific and conserved mechanisms of the polymerase-associated factor 1 complex in replication stress responses.

DNA replication stress threatens genome stability, but how plants respond to this challenge remains unclear. Here we show that the evolutionarily conserved polymerase-associated factor 1 complex (PAF1C) is required for the replication stress response in Arabidopsis. Plants lacking functional PAF1C are hypersensitive to replication stress-inducing agents. Mechanistically, we reveal a plant-specific pathway in which the stress-activated kinase WEE1 phosphorylates PAF1 to prevent its protein degradation, a regulatory mechanism absent in yeast. By contrast, we uncover a conserved pathway in which the replication factor C (RFC) complex recruits PAF1C to stalled replication forks, where PAF1C in turn recruits the E2 ubiquitin-conjugating enzymes UBC1/2 and the E3 ubiquitin ligases HUB1/2 to promote histone H2B monoubiquitination, thereby stabilizing the forks. Collectively, our findings suggest that PAF1 regulates replication stress responses by integrating a plant-specific protein stability control mechanism (WEE1-PAF1) with a conserved recruitment mechanism (RFC-PAF1-UBC1/2-HUB1/2). This work establishes a new function for PAF1C and expands the mechanistic understanding of WEE1 and the RFC complex in the replication stress response.