Figure 2.

Organization of OPN1LW and OPN1MW and spectral tuning of L- and M-cones. OPN1LW and OPN1MW lie in tandem on the X-chromosome at Xq28 and are nearly identical in sequence. Exons are black, yellow, green and red boxes. Exons 1 and 6 are invariant (black boxes), exon 5 (red and green boxes) encodes amino acids that tune the spectra and define whether the encoded pigment is of the L-class or M-class. Exons 2, 3 and 4 specify amino acids that tune the spectra but do not define whether the encoded pigment is of the L- versus M-class. These are indicated using the single letter amino acid code followed by the codon/amino acid position. In this illustration, the amino acids encoded by exons 2, 3 and 4 that shift the spectrum toward the long-wavelength (magnitudes given in figure) are shown in OPN1LW (yellow boxes with red outlines) and those that shift the spectrum to the short-wavelengths are shown in OPN1MW (yellow boxes with green outlines). Depending on the combination of amino acids encoded by exons 2–4 L-cone peak sensitivity ranges from 549 to 559 nanometers (nm). Depending on exon 3 and 4, M-cone peak sensitivity ranges from 530 to 536 nm. The single letter amino acid code is A=alanine, F=phenylalanine, G=glycine, I=isoleucine, L=leucine, P=proline, Q=glutamine, R=arginine, S=serine, T=threonine, Y=tyrosine.