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J. Bellingham; Analysis of melanopsin (opn4) gene loci in Teleost fish: Identification of a fourth melanopsin locus. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3636.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To examine the structure and evolution of melanopsin (opn4) genes in Teleost fish through a combination of in silico and classical molecular biology approaches. Methods: The nucleotide and protein sequences of zebrafish (Danio rerio) opn4c melanopsin isoform and cod (Gaddus morhua) opn4a and opn4b melanopsin isoforms, were used as bait to probe the zebrafish (http://www.ncbi.nlm.nih.gov/genome/seq/DrBlast.html) and pufferfish, Fugu rubripes (http://www.ncbi.nlm.nih.gov/BLAST/Genome/fugu.html) sequence databases using the BLAST algorithms available. Standard molecular techniques were used to refine and confirm the in silico observations in zebrafish. Results:Three isoforms of melanopsin and their tissue localisation have been described in various Teleost species to date – opn4a and opn4b in cod (Drivenes et al., 2003, J. Comp. Neurol. 456, 84–93), and opn4c in zebrafish (Bellingham et al., 2002, Mol. Brain Res. 107, 128–136). In silico analysis has revealed that the Teleost type opn4c melanopsin gene is intronless in zebrafish (and Fugu), whilst the Fugu orthologues of cod type opn4a and opn4b melanopsin have a gene structure broadly conserved with that of human OPN4. In addition to these three melanopsin genes, I have identified a fourth melanopsin locus in the Fugu genome, opn4d, that contains intron positions conserved with human OPN4. Across the transmembrane domains, Fugu opn4d shares 65% amino acid identity with Fugu opn4c, and 52 and 57% identity with Fugu opn4a and opn4b respectively. Phylogenetic analysis indicates that Fugu opn4d segregates with opn4c in the mammalian branch of the melanopsin family. Characterisation and tissue expression of the zebrafish opn4d homologue is currently being undertaken. Conclusions:A search for melanopsin genes in the Teleost genome has led to the identification of opn4d, a fourth melanopsin locus in Fugu. The intronless opn4c is probably a retrogene arising from opn4d under a similar mechanism that has given rise to the intronless rod–opsin gene, rho, in Teleosts (Bellingham et al., 2003, J. Exp. Zool. 297B,1–10).
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