MELANOCYTE-SPECIFIC GENE-EXPRESSION - ROLE OF REPRESSION AND IDENTIFICATION OF A MELANOCYTE-SPECIFIC FACTOR, MSF

For a gene to be transcribed in a tissue-specific fashion, expression must be achieved in the appropriate cell type and also be prevented in other tissues. As an approach to understanding the regulation of tiss ue-specific gene expression, we have analyzed the requirements for mel anocyte-specific expression of the tyrosinase-related protein 1 (TRP-1 ) promoter. Positive regulation of TRP-1 expression is mediated by bot h an octamer-binding motif and an 11-bp element, termed the M box, whi ch is conserved between the TRP-1 and other melanocyte-specific promot ers. We show here that, consistent with its ability to activate transc ription in a non-tissue-specific fashion, the M box binds the basic-he lix-loop-helix factor USF in vitro. With the use of a combination of s ite-directed mutagenesis and chimeric promoter constructs, additional elements involved in regulating TRP-1 expression were identified. Thes e include the TATA region, which appears to contribute to the melanocy te specificity of the TRP-1 promoter. Mutational analysis also identif ied two repressor elements, one at the start site, the other located a t -240, which function both in melanoma and nonmelanoma cells. In addi tion, a melanocyte-specific factor, MSF, binds to sites which overlap both repressor elements, with substitution mutations demonstrating tha t binding by MSF is not required for repression. Although a functional role for MSF has not been unequivocally determined, the location of i ts binding sites leads us to speculate that it may act as a melanocyte -specific antirepressor during transcription of the endogenous TRP-1 g ene.