Defects in DNA Ligase I Trigger PCNA Ubiquitination at Lysine 107

In all eukaryotes, the ligation of newly synthesized DNA, also known as Okazaki fragments, is catalyzed by DNA ligase I1. An individual with a DNA ligase I deficiency exhibited growth retardation, sunlight sensitivity and severe immunosuppression2, likely due to accumulation of DNA damage. Surprisingly, not much is known about the DNA damage response (DDR) in DNA ligase I-deficient cells. Because DNA replication and DDR pathways are highly conserved in eukaryotes, we utilized Saccharomyces cerevisiae as a model system to address this question. We uncovered a novel pathway, which facilitates ubiquitination of lysine 107 of proliferating cell nuclear antigen (PCNA). Unlike ubiquitination at lysine 164 of PCNA in response to UV irradiation, which triggers translesion synthesis3, modification of lysine 107 is not dependent on the ubiquitin conjugating enzyme (E2) Rad64 nor the ubiquitin ligase (E3) Rad185, but requires the E2 variant Mms26 in conjunction with Ubc47 and the E3 Rad58,9. Surprisingly, DNA ligase I-deficient cdc9-1 cells that carry a PCNA(K107R) mutation are inviable, because they cannot activate a robust DDR. Furthermore, we show that ubiquitination of PCNA in response to DNA ligase I-deficiency is conserved in humans, yet the lysine that mediates this modification remains to be determined. We propose that PCNA ubiquitination provides a “DNA damage code” that allows cells to categorize different types of defects that arise during DNA replication.