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. Recognizes the substrate consensus sequence [ST]-Q. Phosphorylates various proteins, which collectively inhibits DNA replication and mitosis and promotes DNA repair and recombination. Phosphorylates grp/CHK1. Phosphorylates 'Ser-137' of histone variant H2AX/H2AV at sites of DNA damage, thereby regulating DNA damage response mechanism. Essential for the DNA damage checkpoint in larval imaginal disks and neuroblasts and for the DNA replication checkpoint in the embryo. Has also an essential role during early nuclear divisions in embryos, where it is required to delay mitosis in response to incomplete DNA replication. Also plays an important role during meiosis, where it may monitor double-strand-break repair during meiotic crossing over, to regulate the progression of prophase I, and to enforce metaphase I delay observed at the end of oogenesis. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with mus304. Expressed both maternally and zygotically. In embryos, during S phase of mid-blastula transition, fails to delay the activation of Cdk1, leads to loss of Rif1 foci, and results in cells entering prematurely into mitosis 13 before the completion of replication resulting in chromosome bridging. Belongs to the PI3/PI4-kinase family. ATM subfamily. Called dATM in some references while it is the ortholog of ATR.