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J. Pahlberg, M. Porter, K.A. Crandall, K. Donner; Amino Acid Substitutions and Spectral Tuning in an Invertebrate Visual Pigment . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3724.
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Our aim was to examine how spectral tuning of an invertebrate long–wavelength–sensitive visual pigment is regulated by changes in opsin amino acid sequence. Phylogenetically all invertebrate opsins constitute an outgroup in relation to all vertebrate opsins.
As a model for invertebrate vision we used adult opossum shrimps of the species Mysis relicta. The absorbance spectrum of the visual pigment of two Finnish populations from different light environments ("Sea" and "Lake") was measured in situ in single rhabdoms by microspectrophotometry (MSP). The two populations have been isolated from each other for ca. 9000 years. RNA and DNA was isolated from individuals of the two different populations and used for cDNA and genomic DNA sequencing.
The wavelengths of maximum absorbance of the Sea and Lake populations were 529 and 554 nm, respectively. The shapes of the absorbance spectra indicated that the pigments were pure porphyropsins (A2 pigments) in both populations. The sequenced genes were characteristic of an opsin protein, including the lysine forming the Schiff base. The sequence difference of the two populations suggests candidate residues for explaining the spectral divergence.
The light sensitivity of the visual pigment depends on interactions between chromophore and opsin. The wavelength of maximum absorbance is determined by the effect of the specific opsin amino acid sequence on the retinal chromophore, the precise location of max depending on interactions between the chromophore and its charged surrounding. Since both Mysis relicta populations use the same chromophore (A2), the difference in spectral sensitivity is due to the differences in the opsin amino acid sequence.
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