DNA ligase I is recruited to sites of DNA replication by an interaction with proliferating cell nuclear antigen: identification of a common targeting mechanism for the assembly of replication factories

In mammalian cells, DNA replication occurs at discrete nuclear sites termed replication factories. Here we demonstrate that DNA Ligase I and the large subunit of replication factor C (RF-C p140) have a homologous sequence of approximately 20 Amino acids at their N-termini that functions as a replication factory targeting sequence (RFTS). This motif consists of two boxes: box 1 contains the sequence IxxFF whereas box 2 is rich in positively charged residues. N-terminal fragments of DNA Ligase I and the RF-C large subunit that contain the RFTS both interact with proliferating cell nuclear antigen (PCNA) in vitro. Moreover, the RFTS of DNA Ligase I and of the RF-C large subunit is necessary and sufficient for the interaction with PCNA. Both subnuclear targeting and PCNA binding by the DNA Ligase I RFTS are abolished by replacement of the adjacent phenylalanine residues within box 1. Since sequences similar to the RFTS/PCNA-binding motif have been identified in Other DNA replication Enzymes and in p21(CIP1/WAF1), we propose that, in addition to functioning as a DNA Polymerase processivity factor, PCNA plays a central role in the recruitment and stable association of DNA replication proteins at replication factories.