Histone H1 and H3 dephosphorylation are differentially regulated by radiation-induced signal transduction pathways

We recently demonstrated that linker histone Hi, which is thought to have a fundamental role in higher-order chromatin structure, becomes transiently dephosphorylated after ionizing radiation (IR) in a mutated ataxia telangie ctasia (ATM) dependent manner. To establish whether Hi dephosphorylation wa s a component of a damage-response pathway that included dephosphorylation of other histones, we asked whether H3 was dephosphorylated in response to IR in a manner similar to H1, H1 and H3 are maximally phosphorylated in met aphase and both are dephosphorylated after IR, However, the duration of IR- induced H3 dephosphorylation is significantly longer than that of IR-induce d Ill dephosphorylation. Moreover, H1 dephosphorylation is ATM-dependent, w hereas H3 dephosphorylation is ATM-independent. These observations suggest that the damage-sensing pathways regulating H3 and H1 dephosphorylation div erge upstream of ATM.