Delivering nonidentical twins

Abstract

Two sibling DNA polymerases synthesize most of the eukaryotic nuclear genome. A new study provides insights into the distinct protein interactions that deliver these replicases for asymmetric leading- and lagging-strand replication and reveals possible cross-talk between DNA replication and other cellular processes.

Figure 1

Comparison of DNA polymerases δ and ε. The four subunits of Pol ε (green) and the three subunits of yeast Pol δ (blue) are shown; a fourth subunit (p12) is present in the human enzyme. The catalytic (Pol3 and Pol2) subunits of both polymerases contain a 3′ exonuclease that can proofread replication errors to achieve the in vitro error rates shown above. The highest error rates are for single base deletions in long homonucleotide runs, which are proofread with low efficiency. Consistent with a role in largely continuous leading-strand replication, Pol ε synthesizes DNA processively, i.e., without dissociating after nucleotide incorporation. Pol δ is highly processive only when assisted by PCNA. Proteins that interact with the individual polymerases are also listed.

Figure 2

The eukaryotic replication fork. Modeled after ref. , but with postreplicative DNA transactions included. The CMG complex (orange) unwinds parental DNA to permit continuous leading-strand synthesis by Pol ε (green). The lagging strand is synthesized as a series of Okazaki fragments that are initiated by an RNA primer (red) made by a primase. This RNA primer is then extended by Pol α to generate a DNA primer that is extended by Pol δ (blue), which in turn synthesizes most of the nascent lagging strand with the assistance of PCNA (yellow ring). Given the weak PCNA–Pol ε interactions, we indicate the possibility of occasional dissociation of PCNA in order to mark the leading strand for postreplicative events that require PCNA. The most frequent of these events is histone deposition to initiate assembly of nucleosomes (purple), which are present every few hundred base pairs. Ribonucleotides (r) are incorporated during replication once every 1,000 to 10,000 bp and are removed by the ribonucleotide excision repair pathway, which could theoretically be coordinated with replication. Least abundant yet highly important are those mismatches generated by the replicases that escape proofreading and that must be removed by mismatch repair to suppress genome instability and tumorigenesis. RPA, replication protein A.