AbstractDNA Polymerase δ plays an essential role in genome replication and in the preservation of genome integrity. InS. cerevisiae, Polδ consists of three subunits: Pol3 (the catalytic subunit), Pol31 and Pol32. We have constructedpol31mutants by alanine substitution at conserved amino acids, and identified three alleles that do not confer any disadvantage on their own, but which suppress the cold-, HU- and MMS-hypersensitivity of yeast strains lacking Pol32. We have shown that Pol31 and Pol32 are both involved in translesion synthesis, error-free bypass synthesis, and in preservation of replication fork stability under conditions of HU arrest. We identified a solvent exposed loop in Pol31 defined by two alanine substitutions at T415 and W417. Whereas pol31-T4l5A compromises polymerase stability at stalled forks,pol31-W417A is able to suppress many, but not all, of the phenotypes arising frompol32Δ. ChIP analyses showed that the absence of Pol32 destabilizes Pole and Polα at stalled replication forks, but does not interfere with checkpoint kinase activation. We show that the Pol31-W417A-mediated suppression of replicationstress sensitivity inpol32Δ stems from enhanced interaction between Pol3 and Pol31, which stabilizes a functional Polδ.