Visualization of uracils created by APOBEC3A using UdgX shows colocalization with RPA at stalled replication forks.

The AID/APOBEC enzymes deaminate cytosines in single-stranded DNA (ssDNA) and play key roles in innate and adaptive immunity. The resulting uracils cause mutations and strand breaks that inactivate viruses and diversify antibody repertoire. Mutational evidence suggests that two members of this family, APOBEC3A (A3A) and APOBEC3B, deaminate cytosines in the lagging-strand template during replication. To obtain direct evidence for the presence of these uracils, we engineered a protein that covalently links to DNA at uracils, UdgX, for mammalian expression and immunohistochemistry. We show that UdgX strongly prefers uracils in ssDNA over those in U•G or U:A pairs, and localizes to nuclei in a dispersed form. When A3A is expressed in these cells, UdgX tends to form foci. The treatment of cells with cisplatin, which blocks replication, causes a significant increase in UdgX foci. Furthermore, this protein- and hence the uracils created by A3A- colocalize with replication protein A (RPA), but not with A3A. Using purified proteins, we confirm that RPA inhibits A3A by binding ssDNA, but despite its overexpression following cisplatin treatment, RPA is unable to fully protect ssDNA created by cisplatin adducts. This suggests that cisplatin treatment of cells expressing APOBEC3A should cause accumulation of APOBEC signature mutations.