CDKs-mediated phosphorylation of PNKP is required for end processing of single-strand DNA gaps on Okazaki Fragments and genome stability

Polynucleotide kinase phosphatase (PNKP) has enzymatic activities as 3' -phosphatase and 5' - kinase of DNA ends to promote DNA ligation and repair. Here, we show that cyclin-dependent kinases (CDKs) regulate the phosphorylation of threonine 118 (T118) in PNKP. This phosphorylation allows recruitment to the gapped DNA structure found in single-strand DNA nicks and/or gaps between Okazaki fragments (OFs) during DNA replication. T118A (alanine)-substituted PNKP-expressing cells exhibited accumulation of single-strand DNA gaps in S phase and accelerated replication fork progression. Furthermore, PNKP is involved in poly (ADP-ribose) polymerase 1 (PARP1)-dependent replication gap filling as a backup pathway in the absence of OFs ligation. Altogether, our data suggest that CDK-mediated PNKP phosphorylation at T118 is important for its recruitment to single-strand DNA gaps to proceed with OFs ligation and its backup errors via the gap-filling pathway to maintain genome stability.