A CELL-SPECIFIC ENHANCER FAR UPSTREAM OF THE MOUSE TYROSINASE GENE CONFERS HIGH-LEVEL AND COPY NUMBER-RELATED EXPRESSION IN TRANSGENIC MICE

The tyrosinase gene encodes the key enzyme of melanin production and i s tightly regulated during development. A yeast artificial chromosome covering the mouse tyrosinase gene has been shown to rescue completely the albino phenotype of recipient mouse strains, conferring copy numb er-dependent, position-independent expression. To investigate the pres ence of cis-acting regulatory elements responsible for the appropriate expression of the tyrosinase gene, DNase I hypersensitive site mappin g was performed. A melanoma cell-specific DNase I hypersensitive site was identified at -12 kb upstream of the tyrosinase gene. Functional a nalysis of the corresponding cis-acting element in transgenic mice and transient transfection assays revealed properties of a strong cell-sp ecific enhancer. RNA expression levels of the transgene correlate with copy number, which is reflected in coat colour and eye pigmentation o f transgenic mice. Full enhancer activity in transient transfections i s obtained with a minimal sequence of 200 bp. Protein binding analysis reveals the presence of a melanoma cell-specific complex which might contribute to the faithful expression of the tyrosinase gene.