Figure 6.—

A model for the interaction between nucleases and checkpoint proteins at cdc13-1-induced damage. According to this model, ssDNA formation begins at the chromosome end (where Cdc13p binds). Exo1, ExoX (which is Rad17, Rad24, Mec3, and Ddc1 dependent), and ExoY contribute to ssDNA production. Rad9 inhibits exonuclease activity by contributing to a nuclease progression barrier centered on X repeats. Exo1 and ExoX are both critical for generating ssDNA beyond the Rad9-dependent barrier. However, if the barrier is missing, due to the absence of Rad9, then ExoX becomes more important than Exo1 for generating ssDNA at single-copy sequences near telomeres. ExoY generates ssDNA in the absence of Exo1 and ExoX.