A SINGLE CIS-ACTING ELEMENT IN A SHORT PROMOTER SEGMENT OF THE GENE ENCODING THE INTERPHOTORECEPTOR RETINOID-BINDING PROTEIN CONFERS TISSUE-SPECIFIC EXPRESSION

Interphotoreceptor retinoid-binding protein (IRBP) is the major protei n component of the interphotoreceptor matrix, IRBP has a highly restri cted tissue-specific expression in retinal photoreceptor cells and in a subgroup of pinealocytes, With the purpose of understanding how tran scriptional regulation contributes to the expression of human IRBP, we have studied a short promoter fragment (from -123 to +18, relative to the transcription start site), We demonstrate, by analysis of the exp ression of the lacZ reporter gene fused to this short promoter fragmen t in transgenic mice, that it is sufficient to confer tissue-specific expression in retinal photoreceptors and in pinealocytes, DNA/protein binding assays, performed to identify binding sites for tissue-specifi c trans-acting factors, have shown that an element between -45 and -58 binds a factor present only in nuclear extracts of retinoblastoma der ived cell lines, which express IRBP. An element further upstream, betw een -86 and -106, binds apparently ubiquitous factors, Site-directed m utagenesis was performed to disrupt a GATTAA motif included in the -45 to -58 binding site and a second inverted GATTAA motif present shortl y upstream. In transgenic mice bearing the mutated version of the prom oter fragment, the expression of the reporter gene was completely abol ished, thus suggesting that this element is essential for tissue-speci fic expression. A GATTAA motif appears in the 5'-flanking regions of s everal photoreceptor-specific genes, suggesting that this could be the recognition site for a photoreceptor-specific factor.