A conserved regulatory element present in all Drosophila rhodopsin genes mediates Pax6 functions and participates in the fine-tuning of cell-specific expression

The Drosophila rhodopsin genes (rh's) represent a unique family of highly regulated cell-specific genes, where each member has its own expression pattern in the visual system. Extensive analysis of the rh's has revealed several functional elements that are involved in cell-specificity. We have investigated the functional role of the RCSI/P3 site that is found in the proximal promoter of all Drosophila rh genes. This sequence is remarkably conserved in evolution and is located 15-30 bp upstream of the TATA box. We have previously shown that, in the context of the rh1 promoter, this element is recognized in vivo by a Pax6 protein, the master regulator of eye development. Thus, rh regulation might represent the ancestral function of Pax6. Here, we investigated the role of the RCSI/P3 sequence in the other rh genes and show that they also mediate Pax6 function. We also tested the potential impact of the various RCSI/P3 sequences on the precise cell-specific expression of rh genes. Our results demonstrate that, even though all RCSI/P3 sequences bind Pax6, they are clearly distinct in various rh promoters and these differences are conserved throughout evolution: RCSI/P3 appears to participate in the fine-tuning of cell-specificity. We also show that Pax6 or a related Pax protein may be involved in the regulation of olfactory genes. Therefore, in addition to performing a global photoreceptor-specific function, RCSI also appears to mediate the combined action of Pax6 and other factors and to contribute to rh regulation in subsets of photoreceptors.