Figure 7.

Model of Sgs1–Top3 and Slx1–4 at the termination of rDNA replication. (Left) When replication forks converge and stall, Sgs1–Top3 helicase–topoisomerase separates the parental strands of the DNA template, allowing replication to finish by a DNA polymerase filling-in reaction (broken lines). (Right) In the absence of Sgs1–Top3, Slx1–4 cleaves the 5′ arm of each replication fork, creating a doubly nicked chromatid and a broken sister chromatid with 3′ overhangs. At the rDNA, this DSB can be repaired by RAD52-independent single-strand annealing (SSA). Arrowheads, 3′-ends.