DIRECT REGULATION OF RHODOPSIN-1 BY PAX-6 EYELESS IN DROSOPHILA - EVIDENCE FOR A CONSERVED FUNCTION IN PHOTORECEPTORS/

Pax-6 is a transcription factor containing both a homeodomain (HD) and a Paired domain (PD). It functions as an essential regulator of eye d evelopment in both Drosophila and vertebrates, suggesting an evolution arily conserved origin for different types of metazoan eyes. Classical morphological and phylogenetic studies, however, have concluded that metazoan eyes have evolved many times independently. These apparently contradictory findings may be reconciled if the evolutionarily ancient role of Pax-6 was to regulate structural genes (e.g., rhodopsin) in p rimitive photoreceptors, and only later did it expand its function to regulate the morphogenesis of divergent and complex eye structures. In support of this, we present evidence that eyeless (ey), which encodes the Drosophila homolog of Pax-6, directly regulates rhodopsin 1 (rh1) expression in the photoreceptor cells. We detect ey expression in bot h larval and adult terminally differentiated photoreceptor cells. We s how that the HD of Ey binds to a palindromic HD binding site P3/RCS1 i n the rh1 promoter, which is essential for rh1 expression. We further demonstrate that, in vivo, P3/RCS1 can be replaced by binding sites sp ecific for the PD of Ey. P3/RCS1 is conserved in the promoters of all Drosophila rhodopsin genes as well as in many opsin genes in vertebrat es. Mutimerized P3 sites in front of a basal promoter are able to driv e the expression of a reporter gene in all photoreceptors. These resul ts suggest that Pax-6/Ey directly regulates rhodopsin 1 gene expressio n by binding to the conserved P3/RCS1 element in the promoter.