MULTIPLE VISUAL PIGMENTS IN A PHOTORECEPTOR OF THE SALAMANDER RETINA

Although a given retina typically contains several visual pigments, ea ch formed from a retinal chromophore bound to a specific opsin protein , single photoreceptor cells have been thought to express only one typ e of opsin. This design maximizes a cell's sensitivity to a particular wavelength band and facilitates wavelength discrimination in retinas that process color. We report electrophysiological evidence that the u ltraviolet-sensitive cone of salamander violates this rule. This cell contains three different functional opsins. The three opsins could com bine with the two different chromophores present in salamander retina to form six visual pigments. Whereas rods and other cones of salamande r use both chromophores, they appear to express only one type of opsin per cell. In visual pigment absorption spectra, the band-width at hal f-maximal sensitivity increases as the pigment's wavelength maximum de creases. However, the bandwidth of the UV-absorbing pigment deviates f rom this trend; it is narrow like that of a red-absorbing pigment. In addition, the UV-absorbing pigment has a high apparent photosensitivit y when compared with that of red- and blue-absorbing pigments and rhod opsin. These properties suggest that the mechanisms responsible for sp ectrally tuning visual pigments separate two absorption bands as the w avelength of maximal sensitivity shifts from UV to long wavelengths.