AN INTERACTION BETWEEN DNA-LIGASE-I AND PROLIFERATING CELL NUCLEAR ANTIGEN - IMPLICATIONS FOR OKAZAKI FRAGMENT SYNTHESIS AND JOINING

Although three human genes encoding DNA ligases have been isolated, th e molecular mechanisms by which these gene products specifically parti cipate in different DNA transactions are not well understood, In this study, fractionation of a HeLa nuclear extract by DNA ligase I affinit y chromatography resulted in the specific retention of a replication p rotein, proliferating cell nuclear antigen (PCNA), by the affinity res in, Subsequent experiments demonstrated that DNA ligase I and PCNA int eract directly via the amino-terminal 118 aa of DNA ligase I, the same region of DNA ligase I that is required for localization of this enzy me at replication foci during S phase, PCNA, which forms a sliding cla mp around duplex DNA, interacts with DNA pol delta and enables this en zyme to synthesize DNA processively, An interaction between DNA ligase I and PCNA that is topologically linked to DNA was detected, However, DNA ligase I inhibited PCNA dependent DNA synthesis by DNA pol delta, These observations suggest that a ternary complex of DNA ligase I, PC NA and DNA pol delta does not form on a gapped DNA template, Consisten t with this idea, the cell cycle inhibitor p21, which also interacts w ith PCNA and inhibits processive DNA synthesis by DNA pol delta, disru pts the DNA ligase I-PCNA complex, Thus, we propose that after Okazaki fragment DNA synthesis is completed by a PCNA-DNA pol delta complex, DNA pol delta is released, allowing DNA ligase I to bind to PCNA at th e nick between adjacent Okazaki fragments and catalyze phosphodiester bond formation.