The mouse ST8Sia II (mST8Sia II/STX) gene encodes a neural cell adhesion molecule-specific polysialic acid synthase whose expression is regulated during the developmental stages of mouse brain. To elucidate the molecular mechanism by which the expression is tissue-specifically and developmentally regulated, we isolated the complete genomic DNA and characterized the promoter of the gene for mST8Sia II. The gene encoding mST8Sia II was found to span about 80 kilobases and to be composed of six exons. Primer extension and S1 nuclease protection analyses revealed that the transcription started from 167 nucleotides upstream of the translational initiation site. Promoter analyses of the 5'-flanking region of the mST8Sia II gene using a luciferase gene reporter system revealed strong promoter activity in retinoic acid-induced differentiated P19 cells, which highly express the mST8Sia II gene. Deletion analyses demonstrated that the minimal promoter activity detected for the proximal region 325 base pairs upstream from the translational initiation codon (-158 to +167) could be modulated by various sequences within the 9. 5-kilobase 5'-flanking region. The minimal promoter was embedded in a GC-rich domain (74%, GC content), in which two Sp1 binding motifs as well as a long purine-rich region were found, but it lacked TATA and CAAT boxes. The positive regulatory region located between -159 and -659 contained two additional Sp1 binding motifs and a long pyrimidine-rich region. We also found that the minimal promoter region of the mST8Sia II gene was sufficient for expression of a reporter gene in mST8Sia II gene-expressing neural differentiated P19 cells but not in nonexpressing ones. Thus the TATA-less GC-rich minimal promoter region of mST8Sia II probably controls the cell type-specific expression of the mST8Sia II gene.