In this work we report that the Saccharomyces cerevisiae RAD9, RAD24, RAD17, MEC1, MEC3 and RAD53 checkpoint genes are required for efficient non-homologous end joining (NHEJ). RAD9 and RAD24 function additionally in this process. Defective NHEJ in rad9Δ–rad24Δ, but not yku80Δ cells, is only partially rescued by imposing G1 or G2/M delays. Thus, checkpoint functions other than transient cell cycle delays may be required for normal levels of NHEJ. Epistasis analysis also indicated that YKU80 and RAD9/RAD24 function in the same pathway for repair of lesions caused by MMS and γ-irradiation. Unlike NHEJ, the checkpoint pathway is not required for efficient site-specific integration of plasmid DNA into the yeast genome, which is RAD52-dependent, but RAD51-independent.
