Excess dormant origins bound by the minichromosome maintenance (MCM) replicative helicase complex play a critical role in preventing replication stress, chromosome instability, and tumorigenesis. In response to DNA damage, replicating cells must coordinate DNA repair and dormant origin firing to ensure complete and timely replication of the genome; how cells regulate this process remains elusive. Herein, we identify a member of the Fanconi anemia (FA) DNA repair pathway, FANCI, as a key effector of dormant origin firing in response to replication stress. Cells lacking FANCI have reduced number of origins, increased inter-origin distances, and slowed proliferation rates. Intriguingly, ATR-mediated FANCI phosphorylation inhibits dormant origin firing while promoting replication fork restart/DNA repair. Using super-resolution microscopy, we show that FANCI co-localizes with MCM-bound chromatin in response to replication stress. These data reveal a unique role for FANCI as a modulator of dormant origin firing and link timely genome replication to DNA repair.

Protein synthesis is crucial for the maintenance of long-term-memory-related synaptic plasticity. The prion-like cytoplasmic polyadenylation element-binding protein 3 (CPEB3) regulates the translation of several mRNAs important for long-term synaptic plasticity in the hippocampus. Here, we provide evidence that the prion-like aggregation and activity of CPEB3 is controlled by SUMOylation. In the basal state, CPEB3 is a repressor and is soluble. Under these circumstances, CPEB3 is SUMOylated in hippocampal neurons both in vitro and in vivo. Following neuronal stimulation, CPEB3 is converted into an active form that promotes the translation of target mRNAs, and this is associated with a decrease of SUMOylation and an increase of aggregation. A chimeric CPEB3 protein fused to SUMO cannot form aggregates and cannot activate the translation of target mRNAs. These findings suggest a model whereby SUMO regulates translation of mRNAs and structural synaptic plasticity by modulating the aggregation of the prion-like protein CPEB3.

The mouse cytoplasmic polyadenylation element-binding protein 3 (CPEB3) is a translational regulator implicated in long-term memory maintenance. Invertebrate orthologs of CPEB3 in Aplysia and Drosophila are functional prions that are physiologically active in the aggregated state. To determine if this principle applies to the mammalian CPEB3, we expressed it in yeast and found that it forms heritable aggregates that are the hallmark of known prions. In addition, we confirm in the mouse the importance of CPEB3's prion formation for CPEB3 function. Interestingly, deletion analysis of the CPEB3 prion domain uncovered a tripartite organization: two aggregation-promoting domains surround a regulatory module that affects interaction with the actin cytoskeleton. In all, our data provide direct evidence that CPEB3 is a functional prion in the mammalian brain and underline the potential importance of an actin/CPEB3 feedback loop for the synaptic plasticity underlying the persistence of long-term memory.

Translesion synthesis polymerases (TLS Pols) are required to tolerate DNA lesions that would otherwise cause replication arrest and cell death. Aberrant expression of these specialized Pols may be responsible for increased mutagenesis and loss of genome integrity in human cancers. The molecular events that control the usage of TLS Pols in non‐pathological conditions remain largely unknown. Here, we show that aberrant recruitment of TLS Polκ to replication forks results in genomic instability and can be mediated through the loss of the deubiquitinase USP1. Moreover, artificial tethering of Polκ to proliferating cell nuclear antigen (PCNA) circumvents the need for its ubiquitin‐binding domain in the promotion of genomic instability. Finally, we show that the loss of USP1 leads to a dramatic reduction of replication fork speed in a Polκ‐dependent manner. We propose a mechanism whereby reversible ubiquitination of PCNA can prevent spurious TLS Pol recruitment and regulate replication fork speed to ensure the maintenance of genome integrity.