A Y-box consensus sequence is required for basal expression of the human multidrug resistance (mdr1) gene

Basal transcription of the human multidrug resistance (mdr1) promoter was studied by chloramphenicol acetyltransferase (CAT) reporter fusion gene analysis in two parental and doxorubicin-resistant human tumor cell lines. Deletion of mdr1 DNA sequences to -89 relative to the start of transcription (at +1) had little effect on expression. Deletion of nucleotide sequences from -89 to -70, however, resulted in a 5-10-fold reduction in mdrCAT expression. DNase I footprint analysis demonstrated that the region from -85 to -70 was protected from nuclease digestion using nuclear extracts from these cell lines. The sequence between -82 and -73 is perfectly homologous with the 10-base pair Y-box consensus sequence found in the promoters of all major histocompatibility complex class-II (MHC II) genes. The Y-box sequence in MHC II genes is required for accurate and efficient transcription and contains the sequence CCAAT in the reverse orientation (Dorn, A., Durand, B., Marfing, C., Le Meur, M., Benoist, C., and Mathis, D. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 6249-6253). Mutations in the reverse CCAAT sequence of the Y-box consensus substantially reduced expression of an mdrCAT vector and eliminated nucleoprotein binding in an electrophoretic mobility shift assay. These results suggest that proteins which bind to the putative Y-box consensus sequence are critical for basal transcriptional regulation of the human mdr1 gene.