A potential cis-regulatory element regulates differential expression of long wavelength-sensitive opsins in zebrafish.

Human long and medium wavelength-sensitive (LWS/MWS) opsins arose from a tandem duplication event resulting in opsins with divergent spectral sensitivities. Zebrafish independently underwent tandem duplication of their lws opsin, resulting in two spectrally divergent genes, lws1/lws2. Thyroid hormone (TH) differentially regulates these opsins; increased TH causes an increase in lws1 expression at the expense of lws2 (“opsin switching”). Here we investigate a cis-regulatory sequence that may mediate the response to TH using promoter-reporter analysis in transgenic zebrafish. We show that the 2.6-kb region upstream of lws1 combined with the 1.8-kb intergenic region are sufficient for typical lws1/2 expression and regulation by TH. We identify the 0.6-kb region upstream of lws1, as the likely region of TH-responsive cis-regulatory elements and use genomic prediction tools to uncover a putative lws regulatory element (LRE), in this 0.6-kb region. When we disrupt the LRE, we find that in larvae, lws1 reporter is prematurely expressed and does not respond to TH, and lws2 reporter increases in response to TH. In juveniles and adults, however, the patterning of lws1/2 reporters largely resembles stereotypical lws1/2 expression. Therefore, the LRE appears to be involved in differential expression of lws opsins and their response to TH during development.