Defects in DNA Ligase I Trigger PCNA Ubiquitination at Lysine 107

Abstract

The anti‐parallel nature of DNA permits continuous synthesis of DNA on the leading strand and discontinuous DNA synthesis on the lagging strand. On the lagging strand, Okazaki fragments are joined by the catalytic action of DNA ligase I. An individual with a DNA ligase I deficiency exhibited growth retardation, sunlight sensitivity and severe immunosuppression, likely due to accumulation of DNA damage. Surprisingly, little is known about the DNA damage response (DDR) in DNA ligase I‐deficient cells. We utilized Saccharomyces cerevisiae as a model system to address this question. We uncovered a novel pathway, which facilitates ubiquitination of proliferating cell nuclear antigen (PCNA) on lysine 107. Unlike PCNA ubiquitination at lysine 164 in response to UV irradiation, which triggers translesion synthesis and is dependent on Rad6 and Rad18, modification at lysine 107 requires the E2 variant Mms2 in conjunction with Ubc4 and Rad5. Surprisingly, DNA ligase I‐deficient cdc9‐1 cells that carry a PCNAK107R mutation are inviable, because they cannot activate a robust DDR. Our data show that depending on the type of damage that a cell encounters, PCNA is ubiquitinated at different lysine residues. We propose a “DNA Damage code” that allows cells to recognize different types of DNA damage through differential PCNA ubiquitination, allowing the cells to activate the appropriate repair response.