Regulation of Translesion Synthesis DNA Polymerase η by Monoubiquitination

DNA polymerase η is a Y family polymerase involved in translesion synthesis (TLS). Its action is initiated by simultaneous interaction between the PIP box in pol η and PCNA and between the UBZ in pol η and monoubiquitin attached to PCNA. Whereas monoubiquitination of PCNA is required for its interaction with pol η during TLS, we now show that monoubiquitination of pol η inhibits this interaction, preventing its functions in undamaged cells. Identification of monoubiquitination sites within pol η nuclear localization signal (NLS) led to the discovery that pol η NLS directly contacts PCNA, forming an extended pol η-PCNA interaction surface. We name this the PCNA-interacting region (PIR) and show that its monoubiquitination is downregulated by various DNA-damaging agents. We propose that this mechanism ensures optimal availability of nonubiquitinated, TLS-competent pol η after DNA damage. Our work shows how monoubiquitination can either positively or negatively regulate the assembly of a protein complex, depending on which substrates are targeted by ubiquitin.