We examined the spatial expression patterns of two orphan receptor guanylyl cyclase genes OlGC4 and OlGC5 during embryogenesis of medaka and characterized the 5′ flanking region required for tissue-specific expression of OlGC4 by introducing promoter–GFP fusion constructs into medaka embryos. Expression of OlGC5 is confined to retinal photoreceptor cells, while OlGC4 is expressed in the retina, pineal organ, and olfactory pits. The OlGC4 upstream region between −2374 and +343 is sufficient to drive the sensory organ-specific gene expression. Mutations in the consensus binding sequences for OTX/CRX transcription factors did not impair the reporter gene expression. Our results suggest that the same isoform of guanylyl cyclase is utilized in both photoreceptors and olfactory cells, and that transcription factors other than OTX/CRX primarily activate the OlGC4 expression.
