Pol31 and Pol32 subunits of yeast DNA polymerase δ are also essential subunits of DNA polymerase ζ

Replication through a diverse array of DNA lesions occurs by the sequential action of two translesion synthesis (TLS) DNA polymerases (Pols), in which one inserts the nucleotide opposite the lesion and the other carries out the subsequent extension. By extending from the nucleotide inserted by another Pol, Pol zeta plays an indispensable role in mediating lesion bypass. Pol zeta comprises the Rev3 catalytic and Rev7 accessory subunits. Pol32, a subunit of the replicative polymerase Pol delta, is also required for Pol zeta-dependent TLS, but how this Pol delta subunit contributes to Pol zeta function in TLS has remained unknown. Here we show that yeast Pol zeta is a four-subunit enzyme containing Rev3, Rev7, Pol31, and Pol32; in this complex, association with Pol31/Pol32 is mediated via binding of the Rev3 C terminus to Pol31. The functional requirement of this complex is supported by evidence that mutational inactivation of Rev3's ability to bind Pol31 abrogates Pol zeta's role in TLS in yeast cells. These findings identify an unexpected role of Pol31 and Pol32 as two essential subunits of Pol zeta, and clarify why these proteins are required for Pol zeta-dependent TLS, but not for TLS mediated by Pol eta in yeast cells. To distinguish the four-subunit complex from the two-subunit Pol zeta, we designate the four-subunit enzyme "Pol zeta-d,"where "-d" denotes the Pol31/Pol32 subunits of Pol delta.