To understand color vision, Nathans and his colleagues had to track down the three opsins embedded in the cell membranes of the three varieties of cones, which absorb short (blue), medium (green), or long (red) wavelengths of light. Rhodopsin, expressed in rod photoreceptor cells, enables animals to see in dim light.
In 1983, Nathans and Hogness published the amino acid sequence of bovine rhodopsin, and a year later they published the human rhodopsin sequence. “We went up quite a few wrong paths.” But four years of labor eventually paid off. “It was hard,” recalls Nathans, now a professor at Johns Hopkins University School of Medicine. “This was a problem that was going to be solved by going directly to the genes.”Īt the time, very few human genes had been cloned, and recombinant DNA methods were crude. He’s heading to the lab of his advisor, David Hogness, where he plans to use the eyeballs, along with a revolutionary new tool called recombinant DNA, to answer a question that had been posed decades before: What is the molecular basis of color vision? “It seemed clear to me that the way to solve these problems was not to study light-absorbing proteins, which are extremely rare, hard to work with, and intermixed with far more abundant proteins,” says Nathans. Beside him jiggles a bucket of cow eyeballs on ice. Jeremy Nathans, then a graduate student at Stanford University, is driving back to campus after visiting a slaughterhouse in San Jose. The work may also point the way to a future in which scientists could treat color blindness by replacing malfunctioning opsin genes, and perhaps, one day, even supercharge humans’ color perception to reveal a new rainbow altogether. The results suggest that the first trichromatic monkey may have been able to respond immediately to its new, more vibrant world-see the ripe fruit among the green buds the red ants on the leaves. But experiments by the Neitzes and others that provide dichromatic animals, such as mice or squirrel monkeys, with an extra opsin are helping to fill in the story of the evolution of human color vision. To understand how that first trichromatic monkey and its similarly equipped primate descendants responded to their heightened sense of sight remains an ongoing quest. “A single nucleotide change can change your color vision.” (See illustration below.) Yet, despite this simplicity, the evolutionary circumstances that allowed our primate ancestors to adopt trichromacy-the three-cone system that gives humans and some other primates the ability to see the world in full-spectrum color-are remarkably intricate. 1 “It’s absolutely stunning,” says Jacobs.
In 1991, Neitz, working with his wife Maureen and their postdoc advisor Jerry Jacobs of the University of California, Santa Barbara, demonstrated that just three amino acid substitutions account for the 30 nm difference in peak absorption between the modern-day red and green cones in humans, with each change shifting the photopigment’s color spectrum by 5 nm to 15 nm. Such a profound expansion of our visual experience actually required very minor genetic alteration. “Adding a third photopigment has been the greatest invention of all, because it multiplies color vision by another 100 times.” If a single opsin gives an animal the ability to distinguish 100 shades, say, the addition of a second opsin, “amazingly, multiplies that by 100,” says color vision researcher Jay Neitz of the University of Washington in Seattle. Adding a third opsin gene doesn’t simply introduce 50 percent more colors its effect is multiplicative.
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