FIGURE 7.

Pif1 directs a small subset of flaps into the two-nuclease pathway for flap processing. A majority of flaps (denoted by the thick arrows) displaced by pol δ are bound and cleaved by FEN1. Continuous displacement of short flaps by pol δ and cleavage by FEN1 eventually produces a nick that LigI seals. On a minority of flaps (denoted by the thin arrows), Pif1 binds the short flap displaced by pol δ prior to FEN1. Pif1 activity lengthens the flap, allowing binding of RPA. RPA inhibits FEN1 but stimulates Dna2. Dna2 then displaces RPA and cleaves the flap, remaining bound to a short RPA-free flap. FEN1 displaces Dna2 and cleaves the short flap, producing the nicked intermediate that LigI seals.