Quantal noise from human red cone pigment

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The rod pigment, rhodopsin, shows spontaneous isomerization activity. This quantal noise produces a dark light of 0.01 photons s−1 rod−1 in human, setting the threshold for rod vision. The spontaneous isomerization activity of human cone pigments has long remained a mystery because the effect of a single isomerized pigment molecule in cones, unlike that in rods, is small and beyond measurement. We have now overcome this problem by expressing human red cone pigment transgenically in mouse rods in order to exploit their large single-photon response, especially after genetic removal of a key negative-feedback regulation. Extrapolating the measured quantal noise of transgenic cone pigment to native human red cones, we obtained a dark rate of 10 false events s−1 cone−1, almost 103-fold lower than the overall dark transduction noise previously reported in primate cones. Our measurements provide a rationale for why mammalian red, green and blue cones have comparable sensitivities, unlike their amphibian counterparts.

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Figure 1: Mouse rods expressing transgenic human red cone pigment.
Figure 2: Responses and action spectrum of mouse rods expressing transgenic red cone pigment.
Figure 3: Estimate of percentage of red cone pigment in transgenic mouse rods.
Figure 4: Expression of human red cone pigment in Rho+/+Gcaps−/− background.
Figure 5: Measurement of spontaneous isomerization rate of red cone pigment.
Figure 6: Background adaptation of wild-type (Rho+/+) mouse rods.


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We thank J. Lai for help in generating the transgene construct, Y. Liang, L. Ding, and Y. Wang for mouse genotyping, J. Chen (University of Southern California School of Medicine) for the Gcaps−/− and Sag−/− mice and J. Lem (Tufts University School of Medicine) for the Rho−/− mice, as well as J. Nathans (Johns Hopkins University School of Medicine) and R. Molday (University of British Columbia) for their gifts of antibodies. We are indebted to P. Ala-Laurila, D. Baylor, and J. Schnapf for discussions. This work was supported by grant EY 06837 from the US National Eye Institute to K.-W.Y.

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Correspondence to King-Wai Yau.

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