Abasic (apurinic/apyrimidinic or AP) sites are a frequent type of DNA damage that threatens genetic stability. The predominant mammalian enzyme initiating repair of AP sites is the Ape1 AP endonuclease (also called Apex or Hap1), which also facilitates DNA binding by several transcription factors (Ref1 activity). We found that expression of the APE1 gene was coordinated with the cell cycle in murine NIH3T3 cells: APE1 mRNA levels rose after the G 1 -S transition and peaked ϳ4-fold higher in early to mid-S phase. The increased APE1 mRNA was the result of transcriptional activation rather than increased mRNA stability. Fusions of various APE1 promoter fragments to the chloramphenicol acetyltransferase CAT reporter gene indicated that APE1 expression depends on two transcription factor Sp1 binding sites within the promoter region. Mutation of these sites or of two CCAAT elements within the APE1 promoter, in conjunction with protein binding studies, demonstrated their specific roles. The Sp1 site upstream of the transcription start, together with an adjacent CCAAT element, establishes a protein-DNA complex required for basal transcription of APE1. The Sp1 site downstream of the transcription start was required for the response to cell growth. Because Ape1 is a dual function enzyme, its cell cycle-dependent expression might affect both DNA repair and the activity of various transcription factors as a function of the cell cycle.Mutagenesis and cell proliferation are essential steps leading to carcinogenesis. DNA damage and error-prone processing cause mutagenesis (1-3). Endogenous DNA damage frequently includes apurinic/apyrimidinic (AP) 1 sites, of which Ͼ10,000/ day are estimated to be formed in each mammalian cell (4). Unrepaired AP sites are cytotoxic lesions that block DNA polymerases, but they are also mutagenic, causing base substitution (5). During DNA repair, AP sites are first incised by an AP endonuclease activity (6). The major mammalian AP endonuclease, Ape1, has been characterized at the biochemical and molecular genetic level by several laboratories (7-11). Ape1 protein also harbors weaker 3Ј-repair diesterase, 3Ј-phosphatase, 3Ј-exonuclease, and RNase H activities (12, 13). The 3Ј-repair diesterase activity is crucial for repair of 3Ј-phosphoglycolate ends at strand breaks generated by ionizing radiation and other oxidative stress (6,14).Ape1 has a second activity as a redox factor (Ref-1) that acts as a reducing donor for oxidized transcription factors such as AP-1 and p53 to restore DNA binding (15, 16). The Ref1 activity depends on a cysteine residue in the Ape1 N-terminal domain unrelated to non-mammalian AP endonucleases of this family; the AP endonuclease activity is located in the conserved Cterminal domain (17,18). In addition to a second, as yet undefined, role in activating p53 (15), human Ape1 protein has also been implicated in the transcriptional regulation of both the parathyroid hormone (19) and of the APE1 gene itself (20) in response to extracellular calcium.The embryonic lethality res...