The multidrug resistance 1 (MDR1) gene encodes a M(r) 170,000 membrane glycoprotein termed P-glycoprotein, which catalyzes the energy-dependent efflux of multiple anticancer agents. We investigated the activation of the MDR1 gene promoter by UV light irradiation in human cancer KB cells after both transient and stable transfection assays of the MDR1 promoter fused to the chloramphenicol acetyltransferase (CAT) gene. Following exposure to UV irradiation, CAT gene expression was about 20-fold increased. A series of promoter dissection analyses showed that two elements extending from -136 to -76 of the 5' flanking sequence and from +1 to +121 of the sequence downstream from the initiation site were required for the stress induction of MDR1 promoter activity. Gel shift assays showed that the specific DNA binding activities of the transacting protein to the MDR1 promoter were augmented in nuclear extracts from the cells treated with UV irradiation. A DNA sequence, an inverted CCAAT box, was identified that specifically bound to this protein, and mutation of this sequence abolished the binding of this protein. Two guanines in the inverted CCAAT box were found to be critical, as methylation of these guanines abrogated the binding. Nuclear run-on assay demonstrated that the transcription level was increased about 5-fold. These results suggest that the activation of the MDR1 promoter may result from transcriptional rather than posttranscriptional events. These studies will provide the basis for understanding the regulatory mechanism for appearance of the drug-resistant phenotype during cancer chemotherapy.