A cis-regulatory element essential for photoreceptor cell-specific expression of a medaka retinal guanylyl cyclase gene.

We examined the spatial expression pattern of an orphan receptor guanylyl cyclase gene, OlGC3, during embryogenesis of medaka, and we analyzed the cis-regulatory region required for its photoreceptor cell-specific expression by introducing promoter-green fluorescent protein gene (GFP) fusion constructs into medaka embryos. Zygotic expression of OlGC3 first appears at stage 36 and is confined to retinal photoreceptor cells. The 5' flanking sequence up to -388 is sufficient to drive the photoreceptor cell-specific gene expression. When a mutation was introduced into the orthodenticle-related homeobox (OTX)-binding consensus sequence at -36/-31, reporter gene expression was completely abolished in retinal photoreceptor cells. The mutation seemed not to impair promoter elements for general transcription factors, because ectopic GFP expression in yolk epithelium was not diminished by the same mutation. These results suggest that the proximal cis-regulatory element containing a consensus binding sequence for OTX transcription factors is essential for OlGC3 expression in photoreceptor cells.