Figure 4.

Sir2p affects both the firing efficiency and distribution of replication origins in the rDNA. Wild-type and sir2Δ hmlΔ cells (E1385) were synchronized in G₁ with α-factor and released into S phase in the presence of HU. Ribosomal DNA fibers were analyzed by DMC as described previously. (A) Representative rDNA fibers. (Red) FISH; (green) BrdU. Bar, 50 kb. (B) Size distribution of BrdU tracks (b) and gaps (a) in sir2Δ rDNA fibers. (C) Model of the distribution of active origins in the rDNA of wild-type and sir2Δ cells (see text).