Arranging eukaryotic nuclear DNA polymerases for replication: Specific interactions with accessory proteins arrange Pols α, δ, and ϵ in the replisome for leading-strand and lagging-strand DNA replication

Abstract

Biochemical and cryo-electron microscopy studies have just been published revealing interactions among proteins of the yeast replisome that are important for highly coordinated synthesis of the two DNA strands of the nuclear genome. These studies reveal key interactions important for arranging DNA polymerases α, δ, and ϵ for leading and lagging strand replication. The CMG (Mcm2-7, Cdc45, GINS) helicase is central to this interaction network. These are but the latest examples of elegant studies performed in the recent past that lead to a much better understanding of how the eukaryotic replication fork achieves efficient DNA replication that is accurate enough to prevent diseases yet allows evolution.

Keywords: DNA polymerases; DNA replication; cryo-electron microscopy; initiation of replication; replisome.

Figure 1

Rearrangement of the MCM double hexamer during helicase activation. The double hexamer that initially surrounds double-strand DNA is reconfigured during activation to yield two active helicases surrounding single stranded DNA. In one mode (left), the Mcm2-7 hexamers become positioned in a C–>N direction on each of the 5′ to 3′ strands and move away from each other. In the other mode (right), the Mcm2-7 hexamers become positioned in a N to C direction on the 5′ to 3′ strands and move past each other toward the fork junctions.

Figure 2

Possible roles for Pol δ and Pol ε in initiating leading-strand DNA replication. In the model shown here, priming of the leading strands is carried out by the CMG-Pol ε-Pol α complex bound to opposite strands (CMG =Mcm2-7-Cdc45-GINS). In the minimal pathway (left), the primer is captured and elongated by PCNA-Pol ε (left). Alternatively (right), the primer is first captured and elongated by PCNA-Pol δ, and after collision of the elongating complex with CMG, Pol δ is ejected and Pol ε then loaded to carry out further elongation. Note that the orientation of the Mcm2-7 helicase is shown as N → C on the 5′ to 3′ strands. An opposite orientation (see Fig. 1) may also support either mechanism for starting leading strand elongation. Not all known accessory factors are shown in this diagram. RPA, replication protein A.