Transcriptional co-regulation of evolutionarily conserved microRNA/cone opsin gene pairs: implications for photoreceptor subtype specification.

The vertebrate retina contains two types of photoreceptor cells, rods and cones, which use distinct types of opsins and phototransduction proteins. Cones can be further divided into several subtypes with differing wavelength sensitivity and morphology. Although photoreceptor development has been extensively studied in a variety of vertebrate species, the mechanism by which photoreceptor subtypes are established is still largely unknown. Here we report two microRNAs (miRNAs), miR-726 and miR-729, which are potentially involved in photoreceptor subtype specification. In the medaka Oryzias latipes, the genes encoding miR-726 and miR-729 are located upstream of the red-sensitive opsin gene LWS-A and the UV-sensitive opsin gene SWS1, respectively, and are transcribed in the opposite direction from the respective opsin genes. The miR-726/LWS pair is conserved between teleosts and tetrapods, and the miR-729/SWS1 pair is conserved among teleosts. in situ hybridization analyses and fluorescence reporter assays suggest that these miRNAs are co-expressed with the respective opsins in specific cone subtypes. Potential targets of miR-726 and miR-729 predicted in silico include several transcription factors that regulate photoreceptor development. Functional analyses of cis-regulatory sequences in vivo suggest that transcription of the paired microRNA and opsin genes is co-regulated by common cis-regulatory modules. We propose an evolutionarily conserved mechanism that controls photoreceptor subtype identity through coupling between transcriptional and post-transcriptional regulations.