Serine/threonine protein kinase which activates checkpoint signaling upon genotoxic stresses such as ionizing radiation (IR), ultraviolet light (UV), or DNA replication stalling, thereby acting as a DNA damage sensor (By similarity). Recognizes the substrate consensus sequence [ST]-Q (By similarity). Phosphorylates various proteins, which collectively inhibits DNA replication and mitosis and promotes DNA repair and recombination (By similarity). Prevents mitotic catastrophe by functioning in the S-phase checkpoint and cooperating with atm-1 in the checkpoint response to double-strand breaks (DSBs) after ionizing radiation (IR) to induce cell cycle arrest or apoptosis via the cep-1/p53 pathway (PubMed:12747829, PubMed:16319925). In response to ionizing radiation, probably required for the association between the brc-1-brd-1 heterodimer and rad-51 and let-70 in order to activate E3-ubiquitin ligase activity of the heterodimer and induce ubiquitination at DNA damage sites (PubMed:16628214). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Recruited to stalled replication forks by rpa-1-coated single-stranded DNA. Recruited to processed double-strand breaks in a rpa-1 and mre-11-dependent manner. Expressed at all larval stages and is expressed at higher level in the adult. RNAi-mediated knockdown results in DNA damage repair defects following ionizing radiation with reduced ubiquitination at DNA damage sites. Belongs to the PI3/PI4-kinase family. ATM subfamily.