Despite the biological importance of UV vision, its molecular bases are not well understood. Here, we present evidence that UV vision in vertebrates is determined by eight specific amino acids in the UV pigments. Amino acid sequence analyses show that contemporary UV pigments inherited their UV sensitivities from the vertebrate ancestor by retaining most of these eight amino acids. In the avian lineage, the ancestral pigment lost UV sensitivity, but some descendants regained it by one amino acid change. Our results also strongly support the hypothesis that UV pigments have an unprotonated Schiff base-linked chromophore.
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