MBE Advance Access originally published online on January 12, 2005
Molecular Biology and Evolution 2005 22(4):1001-1010; doi:10.1093/molbev/msi086
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Research Article |
Reconstitution of Ancestral Green Visual Pigments of Zebrafish and Molecular Mechanism of Their Spectral Differentiation
Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Chiba, Japan
E-mail: kawamura{at}k.u-tokyo.ac.jp.
We previously reported that zebrafish have four tandemly duplicated green (RH2) opsin genes (RH2-1, RH2-2, RH2-3, and RH2-4). Absorption spectra vary widely among the four photopigments reconstituted with 11-cis retinal, with their peak absorption spectra (
max) being 467, 476, 488, and 505 nm, respectively. In this study, we inferred the ancestral amino acid (aa) sequences of the zebrafish RH2 opsins by likelihood-based Bayesian statistics and reconstituted the ancestral opsins by site-directed mutagenesis. The ancestral pigment (A1) to the four zebrafish RH2 pigments and that (A3) to RH2-3 and RH2-4 showed max at 506 nm, while that (A2) to RH2-1 and RH2-2 showed a max at 474 nm, indicating that a spectral shift had occurred toward the shorter wavelength on the evolutionary lineages A1 to A2 by 32 nm, A2 to RH2-1 by 7 nm, and A3 to RH2-3 by 18 nm. Pigment chimeras and site-directed mutagenesis revealed a large contribution (
15 nm) of glutamic acid to glutamine substitution at residue 122 (E122Q) to the A1 to A2 and A3 to RH2-3 spectral shifts. However, the remaining spectral differences appeared to result from complex interactive effects of a number of aa replacements, each of which has only a minor spectral contribution (1–3 nm). The four zebrafish RH2 pigments cover nearly an entire range of max distribution among vertebrate RH2 pigments and provide an excellent model to study spectral tuning mechanisms of RH2 in vertebrates.
Key Words: zebrafish • RH2 opsins • visual pigments • gene duplication • spectral differentiation • ancestral sequence
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