Identification of a cis-acting element that enhances the pigment cell-specific expression of the human tyrosinase gene.

To identify the cis-acting element that is responsible for the pigment cell-specific expression of the human tyrosinase gene, we analyzed the promoter activity of its 5'-flanking region by transient expression assays. The fusion genes were constructed by inserting the 5'-flanking region of the human tyrosinase gene upstream from the firefly luciferase gene and were introduced into human melanoma cells and HeLa cells. We thus found the element, located between 2.7 and 1.8 kilobase pairs upstream from the transcription initiation site, that enhances the transient expression of the luciferase reporter gene in melanoma cells, but not in HeLa cells, the tyrosinase gene expression of which is not detectable. Using the fusion genes containing putative enhancer elements under the control of the heterologous simian virus 40 promoter, we identified the pigment cell-specific enhancer of approximately 200 base pairs (bp) between -2.0 and -1.8 kilobase pairs and localized the core sequence to a 39-bp region. This 39-bp core element was then confirmed to direct the melanoma cell-specific expression of the reporter gene under the tyrosinase gene promoter. We thus propose that this core element is responsible for the pigment cell-specific expression of the human tyrosinase gene.