Fig. 1.

DSB hotspots compete with each other over ∼200-kb regions. (A) Rec12 (Spo11 homolog; green ball) forms DSBs, remains covalently linked to 5′ ends, and is removed by an endonuclease to expose 3′ single-stranded tails (30, 58). Tails invade homologous DNA to form joint DNA molecules, resolved to form cross-overs as shown or non–cross-overs. Each line is ssDNA, blue and red from each parent; dashed lines indicate newly synthesized DNA. (B) DSBs are reduced at hotspots within ∼100 kb of the Rec25, Rec27-dependent ade6-3049 hotspot (4, 10, 24). Shown is DSB frequency, measured by ChIP-chip of Rec12-DNA covalent complexes, on part of Chr 3 with (ade6-3049; red line) or without (ade6-3057; blue line) a hotspot. (C) DSB frequency is increased only on the chromosome with hotspot alteration. (Upper) Natural hotspot mbs1 on Chr 1 (mbs1⁺ vs. mbs1-20 deletion). (Lower) ade6 hotspot on Chr 3 (ade6-3049 vs. ade6-3057). (SI Appendix, Fig. S1) (11). Individual points (+) are microarray values at other hotspots ≤50 kb of each side of the compared hotspot. Heat maps (densest in magenta) indicate densities of other points. Scales, log₁₀. (D) Competition extends ∼100 kb on each side of a hotspot. Hotspot peaks surrounding mbs1 or ade6-3049 were integrated in the presence or absence of mbs1 or ade6-3049; each hotspot’s DSB ratio was plotted against its distance from mbs1 or ade6-3049. Values >1 indicate more breakage in the absence of each hotspot. Data were averaged (blue line) using a 50-kb sliding window in 25-kb steps. Median ratio is 0.95 (solid red line); dashed red lines indicate median ± two SD.