The cDNA of a new human mesangium-predominant gene, megsin, a novel member of the serpin superfamily, has recently been cloned. This study investigates the regulatory mechanisms of megsin gene expression. A genomic clone of the human megsin gene was obtained by screening bacterial artificial chromosome (BAC) library with the megsin cDNA. The analysis for exon-intron junctions of megsin genomic DNA demonstrated that the gene contained 8 exons and 7 introns, spanned 20 kbp, and that the genomic structure of the serpin superfamily was highly conserved. Fluorescence in situ hybridization (FISH) revealed that the megsin gene is localized in chromosome 18q21.3, close to the other serpin genes. The transcriptional start site, located by primer extension analysis, was 391 bp upstream from the start codon. The sequence and reporter analyses on 4021-bp-long 5'-flanking region of megsin gene demonstrated a consensus promoter segment within this region and a relatively strong promoter activity in human mesangial cells and A431, a human tumor cell line recently reported to express a novel serpin identical with megsin. Moreover, this study utilized site-directed and deletion mutagenesis analyses, and electrophoretic mobility shift assay identified one positive regulatory motif, an incomplete activator protein-1 (AP-1) binding motif (CTGATTCAC) within the -120 to -112 region. This cis-acting element in the 5'-flanking region of megsin is involved in the activation of the megsin gene in mesangial cells.