Mediator complex subunit 1 (Med1)/Thyroid hormone receptor-associated protein 220 (TRAP220),an essential component of thyroid hormone receptor-associated proteins (TRAP)/mediator,plays important roles in hormone responses and tumorigenesis.However,the role of Med1 in the DNA damage response has not been studied.In this study,we found that DNA damage,resulted from γ-irradiation,ultraviolet (UV)-irradiation,or hydroxyurea,induced phosphorylation of Med1 in vivo.Phosphorylation of Med1 was abrogated by either caffeine or wortmannin treatment,suggesting that Med1 is phosphorylated through the DNA damage checkpoint pathway.A checkpoint kinase 1 (Chk1)/checkpoint kinase 2 (Chk2) consensus phosphorylation motif was identified at Serine 671 of Med1 and Ser671 motif was primarily phosphorylated by Chk2 in vitro.Moreover,the in vivo phosphorylation of Med1 was abrogated by a Chk2 inhibitor,and physical interaction between Chk2 and Med1 was observed,confirming that Chk2 is responsible for Med1 phosphorylation upon DNA damage.These results suggest that Med1 is a novel target for the DNA damage checkpoint pathway and may participate in the DNA damage response.Consistent with this notion,knockdown of Med1 expression caused a significant increase in cellular sensitivity to UV irradiation.Moreover,microarray analysis revealed that the UV-induced activation of the transcription of important regulators of cell cycle control and DNA repair,including p21,Gadd45,Rad50,DnaJ,and RecQL,was impaired upon Med1 knockdown.Taken together,our data suggest that Med1 is a novel target for Chk2-mediated phosphorylation and may play a role in cellular DNA damage responses by mediating proper induction of gene transcription upon DNA damage.