PCNA is required for initiation of recombination-associated DNA synthesis by DNA polymerase delta

Abstract

Genetic recombination ensures proper chromosome segregation during meiosis and is essential for genome stability and tumor suppression. DNA synthesis after Rad51-mediated DNA strand invasion is a crucial step during recombination. PCNA is known as the processivity clamp for DNA polymerases. Here, we report the surprising observation that PCNA is specifically required to initiate recombination-associated DNA synthesis in the extension of the 3' end of the invading strand in a D loop. We show using a reconstituted system of yeast Rad51, Rad54, RPA, PCNA, RFC, and DNA polymerase delta that loading of PCNA by RFC targets DNA polymerase delta to the D loop formed by Rad51 protein, allowing efficient utilization of the invading 3' end and processive DNA synthesis. We conclude that PCNA has a specific role in the initiation of recombination-associated DNA synthesis and that DNA polymerase delta promotes recombination-associated DNA synthesis.

Figure 1. PCNA/RFC stimulate Pol η in recombination-associated DNA synthesis and are necessary for Pol δ-dependent DNA synthesis during D-loop extension

(A) Schematic representation of the reconstituted recombination reaction (top) and the assays (bottom) using either a 5'-end-labeled invading 95-mer or α-³²P-dCTP incorporation to identify DNA synthesis products. (B) α-³²P-dCTP incorporation assay. Reactions were either complete (Pol η, lanes 2–5; Pol δ, lanes 6–9) or lacked PCNA/RFC (Pol η, lanes 10–13; Pol δ, lanes 14–17) using the AatII-95-mer. Lane 1 contains a D-loop reaction with 5'-end-labeled AatII-95-mer as a marker (M), and the positions of the 95-mer and D-loop are indicated. The signal above the extended D-loops in lanes 3–5 and 7–9 represents products of rolling circle replication with greater than full-length synthesis products (see Fig. 3D) that also depend on DNA strand invasion (Fig. 2B). The signal running just above the 95-mer marker in lanes 3–5 and 7–9 represents dissociated D-loops, which form during sample processing and electrophoresis (see Supplemental Figure 6). The signal co-migrating with the 95-mer in lanes 10–17 is due to self-priming of the 95-mer, and was particularly evident with Pol η (see also Fig. 2B, C). (C) D-loop extension by Pol δ with 5'-end-labeled AatII-95-mer. Reactions were either complete (lanes 1–4), lacking PCNA/RFC (lanes 5–8), lacking RPA (lanes 9–12), or lacking PCNA/RFC and RPA (lanes 13–16). (D) Quantitation of the results in (C) and additional experiments. Shown are means from at least three independent experiments and one standard deviation, except for the reactions containing Pol δ only, which were repeated twice. (E) Efficiency of D-loop extension is independent of sequence context. The relative position of the homologies on the duplex DNA target for the AatII- and PstI-95-mers is schematically indicated. Reactions were complete with either Pol η (lanes 2–5, 11–14) or Pol δ (lanes 6–9, 15–18) with 5'-end-labeled invading ssDNA, either the AatII-95-mer (lanes 1–9) or the PstI-95-mer (lanes 10–18). Lanes 1 and 10 were controls with complete reactions lacking only Rad54 and polymerase. (F) Quantitation of the results in (E) and additional experiments. Shown are means from at least three independent experiments and one standard deviation.

Figure 2. Recombination-associated DNA synthesis depends on Rad51, Rad54, and homology

(A) Scheme of the α-³²P-dCTP incorporation assay. (B) D-loop extension requires Rad51 and Rad54. α-³²P-dCTP incorporation assays were complete with either Pol η or Pol δ (lanes 2–5), omitted Rad54 (lanes 6–9), Rad51 (lanes 10–13), or RPA, Rad51 and Rad54 (lanes 14–17). Lane 1 contains a D-loop reaction with 5'-end-labeled AatII-95-mer as a marker (M), and the positions of the 95-mer and D-loop are indicated. (C) D-loop extension requires the Rad54 ATPase activity and homology. Reactions were complete with either Pol η or Pol δ and wild type Rad54 protein (lanes 2–5) or the ATPase-deficient Rad54-K341R protein (lanes 6–9). In lanes 10–13, reactions using a heterologous 95-mer (olWDH640) were analyzed. Lane 1 contains size markers as described in (B). The signals representing rolling-circles, dissociated D-loops and primer self-priming were described in the legend to Figure 1B.

Figure 3. Topological constraint limits DNA synthesis

(A) Scheme of the assay with the 5'-end-labeled 95-mer. (B) Extension of the invading 3'-end in a D-loop by Pol δ requires PCNA. 2-dimensional product analysis assays with 5'-end-labeled invading ssDNA (AatII-95-mer). D-loop extension products in complete reactions (10 min time point) with Pol δ (left) or omitting PCNA/RFC (right) were separated in the first dimension by native electrophoresis and in the second dimension under denaturing, alkaline conditions. Quantitation of the substrate and product species is provided in % of total. (C) Product length is limited by topological constraint. Complete reactions with Pol δ were carried out with a titration of RPA (0–1 μM) using end-labeled AatII-95-mer. Eukaryotic topoisomerase I (lanes 4–6, 10–12, 16–18) or buffer control (lanes 1–3, 7–9, 13–15) was added at 2.5 min after addition of Pol δ and products were analyzed on native gels. (D) 2-dimensional product analysis. Products of the 10 min time points of reactions containing 1 μM RPA (see lanes 15, 18 in C) supplemented (right) or not (left) with topoisomerase I were analyzed by 2-dimensional gel electrophoresis as in B. Quantitation of the different substrate and product species is provided as % of total. The positions of full-length extension products (2.7 kb) and rolling circle products (>2.7 kb) are indicated.

Figure 4. Order-of-addition experiments

(A) Scheme of the order-of-addition experiments. (B) Complete α-³²P-dCTP incorporation assays with the AatII-95-mer containing Pol δ were staged by the different protocols schematically indicated on the left hand side. Lane 1 contains a D-loop reaction with end-labeled AatII-95-mer as size markers (M), and the positions of the 95-mer and D-loop are indicated. The signals representing rolling-circles, dissociated D-loops and primer self-priming were described in the legend to Figure 1B. (C) PCNA targets DNA polymerase δ to the 3'-OH end of the invading strand in a recombination-mediated D-loop. Working model depicting the sequence of DNA strand invasion and D-loop extension by Pol δ in the presence of PCNA/RFC. The topological constraint limiting the extent of DNA synthesis can be relieved by the addition of Topo I. The question mark indicates a possible pathway leading to D-loop migration, which would require additional activities. For more discussion see text.