Estrogen receptor ␣ (ER␣) interacts with basal transcription factors, coregulatory proteins, and chromatin modifiers to initiate transcription of the target genes. We have identified a novel interaction between ER␣ and the DNA repair protein 3-methyladenine DNA glycosylase (MPG) thereby providing a functional link between gene expression and DNA repair. Interestingly, the ER␣-MPG interaction was enhanced by the presence of estrogen response element (ERE)-containing DNA. In vitro pull-down assays indicated that the interaction of ER␣ with MPG was direct and occurred through the DNAand ligand-binding domains and the hinge region of the receptor. More importantly, endogenously expressed ER␣ and MPG from MCF-7 cells coimmunoprecipitated with ER␣-and MPG-specific antibodies. The ER␣-MPG interaction had functional consequences on the activities of both proteins. ER␣ increased MPG acetylation, stabilized the binding of MPG with hypoxanthine-containing oligos, and enhanced MPG-catalyzed removal of hypoxanthine from DNA. In turn, MPG dramatically stabilized the interaction of ER␣ with ERE-containing oligos, decreased p300-mediated acetylation of the receptor, and reduced transcription of simple and complex ERE-containing reporter plasmids in a dose-dependent manner. Our studies suggest that recruitment of MPG to ERE-containing genes influences transcription and plays a role in maintaining integrity of the genome by recruiting DNA repair proteins to actively transcribing DNA.Estrogen is critical for the growth, development, and homeostasis of neural, skeletal, cardiovascular, and reproductive tissues (1, 2). The actions of estrogen are mediated by two members of the nuclear receptor family, estrogen receptors (ERs) 1 ␣ and ␤. ER␣ is, however, generally a more potent transcriptional activator than ER␤ (3-5). Like other nuclear receptor family members, ER␣ has a modular structure. At the amino terminus is the A/B region with its autonomous activation function 1 (6). Region C encompasses the DNA binding domain and is linked by the hinge domain to region E, which contains the ligand-binding domain (LBD, Ref. 7). The DNAbinding domain has two zinc finger motifs that are involved in DNA binding. The LBD contains a hydrophobic pocket that interacts with estrogens and antiestrogens. The LBD also contains a ligand-dependent activation function 2, which is responsible for interaction of the receptor with coregulatory proteins (8 -10).ER␣ binds to estrogen response elements (EREs) in target genes to initiate changes in transcription. The consensus ERE is comprised of the palindromic sequence GGTCAnnnTGACC and is found in the Xenopus laevis vitellogenin A2 gene (11). In addition to interacting with EREs, ER␣ modulates transcription through its interaction with components of the basal transcription machinery, regulatory proteins, and chromatin modifiers (12). ER␣ interacts with proteins in the basal transcription complex including TATA-binding protein (13). The coregulatory proteins steroid receptor coactivator 1, transcription interm...