Pol32 phosphorylation strengthens RPA binding and promotes mutagenic break‑induced replication in yeast

A short, phosphorylation‑sensitive stretch of the Pol32 polymerase subunit helps determine when cells deploy a risky but sometimes necessary DNA‑repair pathway, new research shows.

In a study published April 9 in Nature Communications, researchers at the University of Sussex report that phosphorylation of two adjacent threonine residues in an RPA‑binding module (RBM) of Pol32 increases the protein’s affinity for replication protein A (RPA) and makes an error‑prone repair pathway called break‑induced replication (BIR) more likely to proceed.

Key findings

• The team identified an RPA‑binding motif in the nonessential Pol32 subunit of DNA polymerase δ in budding yeast (Saccharomyces cerevisiae).
• Phosphorylation at Thr256 and Thr257 increases Pol32’s binding to Rfa1, the largest subunit of yeast RPA, in biochemical assays.
• Phospho‑mimetic substitutions at those residues strengthen the interaction and enhance BIR efficiency in vivo using a chromosomal BIR reporter.

How BIR fits into DNA repair

BIR is a mechanism that repairs one‑ended DNA breaks, such as those generated when a replication fork collapses or when telomeres erode. Instead of rejoining two broken ends, the exposed end invades an intact DNA template and copies long tracts of sequence. While BIR can rescue otherwise lethal lesions, DNA synthesis during BIR is highly mutagenic and has been linked to copy‑number gains, large genome rearrangements and alternative telomere maintenance—features commonly observed in cancer genomes.

What the researchers did

Using yeast genetics, mutational analysis, biochemical binding assays and structural modeling, the authors mapped the RBM in Pol32 that contacts Rfa1 and tested how phosphorylation affected the interaction. Co‑immunoprecipitation and fluorescence‑polarization experiments showed increased affinity when the Pol32 RBM was phosphorylated. Kinase assays and molecular models supported a plausible fit of a phosphorylated RBM into a conserved RPA interaction surface, resembling contacts seen in other RBM‑containing proteins.

Interpretation and broader relevance

The Sussex team propose that Pol32 functions as a phosphorylation‑regulated “rheostat” for BIR: when the RBM is unmodified, polymerase δ interacts only weakly with RPA‑coated single‑stranded DNA at broken forks, reducing BIR engagement; phosphorylation tunes the interaction upward, favoring BIR initiation and continuation.

Although the experiments were performed in budding yeast, the authors note potential conservation. POLD3, the human ortholog of Pol32, participates in replication restart and BIR‑like events (including mitotic DNA synthesis) in mammalian cells, and human RPA retains structural similarity to yeast Rfa1. That raises the possibility that phosphorylation of a POLD3 RBM or another polymerase subunit could modulate BIR‑like repair in human cells—an idea with implications for tumor cells that experience chronic replication stress or that use alternative telomere maintenance.

Open questions

The study does not definitively identify the kinase(s) responsible for Thr256/Thr257 phosphorylation in vivo, nor does it establish the precise signals or cell‑cycle stages that trigger modification. Large‑scale phosphoproteomic datasets and follow‑up work in mammalian systems will be needed to test conservation and physiological regulation.

Publication and access

The paper, led by first authors David Jones and Rowin Appanah with corresponding authors Antony W. Oliver and Ulrich Rass at the Genome Damage and Stability Centre (University of Sussex), is open access and includes source data for the assays reported. The supplementary materials provide a roadmap for laboratories seeking candidate RBMs in POLD3 and other polymerase subunits.

Why it matters

By pinpointing a short, phosphorylation‑sensitive motif that tunes Pol32’s engagement with RPA, the work adds a mechanistic layer to how cells decide whether to execute a hazardous but sometimes lifesaving repair program. Understanding that switch could help explain how genome rearrangements arise and might reveal new targets for cancer research focused on replication stress and telomere maintenance.