Purpose: Ataxia telangiectasia (AT) is an autosomal recessive disorder associated with radiation sensitivity and an increased incidence of leukemia, lymphoma, and some solid tumors. After exposure to ionizing radiation, cells from patients with AT demonstrate an attenuated G1-phase checkpoint. Because c-jun is known to regulate, in part, the exit from G1 and the onset of DNA replication, we analyzed c-jun transcription in irradiated AT fibroblasts. Methods and Materials: AT5BI fibroblasts were irradiated and RNA was extracted and assayed for c-jun expression by Northern blot analysis. Transcriptional regulation of c-jun was evaluated by use of the 5' untranslated region of the jun promoter linked to the chloramphenicol acetyl transferase (CAT) reporter gene. Deletion mutants of the RSRF, SP-1, AP-1, and CCAAT domains within the jun promoter linked to the CAT reporter were transfected into AT5BI cells. Transfectants were irradiated, and CAT expression was quantified. After x-irradiation, nuclear protein binding to CCAAT was evaluated by an electrophoretic mobility shift assay. Results: X- ray-mediated c-jun expression was sustained in AT5BI cells as compared to only transient expression in irradiated normal diploid fibroblasts. Mutation of either the AP-1 or CCAAT domains within the c-jun promoter reduced transcription by 50% and combined deletion of both AP-1 and CCAAT cis-acting elements entirely eliminated radiation-mediated transcriptional activation. Electrophoretic mobility gel shift assay of the nuclear proteins isolated from irradiated AT fibroblasts demonstrated their increased binding to the CCAAT sequence at 30 min after irradiation. Competition for nuclear protein binding to the CCAAT sequence with excess cold CCAAT demonstrated that protein binding to this sequence was specific. These findings were distinct from induction by phorbol esthers in that the RSRF cis-acting element and DNA segments upstream of -132 base pairs do participate in c-jun induction by phorbol esthers but not by radiation. Conclusions: Radiation-mediated transcriptional regulation of c-jun is prolonged in AT fibroblasts and is regulated in combinatorial control by the AP-1 and CCAAT domains, and transcriptional regulation is distinct from that induced by phorbol esthers.