Strand displacement by DNA polymerase δ In S. cerevisiae, DNA polymerase δ is a complex consisting of three subunits: Pol3, Pol31, and Pol32 (Xxxxx et al. 1998). Pol3 is the catalytic subunit and it contains both polymerase and exonuclease (3’-5’) activities (Xxxxxxx and Burgers, 2016). Furthermore, all three subunits contain PIP-box motifs (QxxhxxFF) for interacting with PCNA (Xx et al. 2002; Xxxxxxx et al. 2011). Interactions with PCNA have been shown to increase the processivity of both DNA polymerase δ and DNA polymerase ε in vitro (Xxxxxxxx et al. 2007; Xxxxxx et al. 2017). During replication on the lagging strand, PCNA is loaded onto primer termini by RFC in a reaction that requires the hydrolysis of ATP (Xxxxxxxxx and Xxxxxxxx, 1990). DNA polymerase δ interacts with PCNA to extend primers synthesised by DNA polymerase α/primase until it reaches the 5’-end of a downstream Okazaki fragment, at which point it begins to displace the RNA/DNA. Importantly, the rate of synthesis decreases with each displaced nucleotide. This ensures that the flaps generated by the polymerase do not become too long for downstream processing (Burgers and Xxxxxx, 2017). In addition, DNA polymerase δ has been shown to switch between its polymerase and nuclease activities, leading to a cycle of continuous elongation and degradation of the nascent DNA (Burgers, 2009). This process has been termed ‘idling’ and is another mechanism through which the polymerase is restricted from excessive strand displacement. Unlike DNA polymerase δ, DNA polymerase ε is very poor at strand displacement synthesis (Xxxxxxxxx et al. 2014; Xxxx et al. 2004; Xxxxxx et al. 1993). Recently, it was shown that DNA polymerase ε is unable to carry out extensive strand displacement synthesis unless its 3’-5’ exonuclease activity is first removed (Xxxxx et al. 2016).
