Identification of cis-acting elements repressing blue opsin expression in zebrafish UV cones and pineal cells.

Opsin genes are expressed in a cell type-specific manner in the retina and the pineal organ for visual and nonvisual photoreceptive purposes, but the regulatory mechanism behind the tissue and cell selectivity is not well understood. In this study, we focus on the expression regulation of the blue-sensitive opsin gene SWS2 of zebrafish by taking a transgenic approach using the green fluorescence protein as an expression reporter. The zebrafish SWS2 is a single-copy gene and is expressed specifically in the "long single cones" in the retina. We found the following. 1) A 0.3-kb region between 0.6 and 0.3 kb 5' of the SWS2 initiation codon, encompassing four cone-rod homeobox-binding sites (OTX sequences), contains the region necessary and sufficient to drive gene expression in long single cones. 2) A 15-bp portion (-341 to -327) in the 0.3-kb region represses the gene expression in the "short single cones," which are dedicated to the UV-sensitive opsin gene SWS1. 3) An 11-bp sequence TAACTGCCAGT (-441 to -431) in the 0.3-kb region, with its adjacent OTX element, also works as a repressor for gene expression in the pineal cells. 4) Finally, this OTX site is necessary for expression repression in the bipolar cells in the retina. These findings open a way for understanding the complex interaction of positive and negative regulatory factors that govern the cell type specificity of the opsin gene expression in the photoreceptive cells in the retina and the pineal organ. We termed the novel 11-bp sequence as the pineal negative regulatory element, PINE.