The histone chaperone Asf1 and the checkpoint kinase Rad53 are found in a complex in budding yeast cells in the absence of genotoxic stress. Our data suggest that this complex involves at least three interaction sites. One site involves the H3-binding surface of Asf11 with an as yet undefined surface of Rad53. A second site is formed by the Rad53-FHA1 domain binding to Asf1-T(270) phosphorylated by casein kinase II. The third site involves the C-terminal 21 amino acids of Rad53 bound to the conserved Asf1 N-terminal domain. The structure of this site showed that the Rad53 C-terminus binds Asf1 in a remarkably similar manner to peptides derived from the histone cochaperones HirA and CAF-I. We call this binding motif, (R/K)R(I/A/V) (L/P), the AIP box for Asf1-Interacting Protein box. Furthermore, C-terminal Rad53-F(820) binds the same pocket of Asf1 as does histone H4-F(100). Thus Rad53 competes with histones H3-H4 and cochaperones HirA/CAF-I for binding to Asf1. Rad53 is phosphorylated and activated upon genotoxic stress. The Asf1-Rad53 complex dissociated when cells were treated with hydroxyurea but not methyl-methane-sulfonate, suggesting a regulation of the complex as a function of the stress. We identified a rad53 mutation that destabilized the Asf1-Rad53 complex and increased the viability of rad9 and rad24 mutants in conditions of genotoxic stress, suggesting that complex stability impacts the DNA damage response.