of their DNA sequence; the differences reside in the terminal regions. Potassium channels with different terminal regions are found in different parts of the brain. Diversity may also arise by another means: several of the relatively short sequences may coalesce in various ways to form a single potassium channel.

The research team of Lily Yeh Jan, at Howard Hughes Medical Institute and the Departments of Physiology and Biochemistry at the University of California, San Francisco, set out to test this possibility. They began by injecting an assortment of genetic material for potassium channels into developing egg cells of Xenopus, the African toad. The potassium channels thus created displayed an interesting combination of traits. For example, one set had lost most of the important “inactivation” function: not only did these channels remain open longer than usual but they were liable to open at any time rather than in precise response to a change in electrical polarization of the nerve cell. (Inactivation itself is thought to come about by a sort of molecule-sized tetherball, which swings into the mouth of the open potassium channel and thereby blocks it; negative charges at the mouth of the pore would temporarily hold the particle, which is presumably positively charged, in place. This model is being tested in several laboratories.) Other channels remained open for some time, but only in the first half of the depolarization phase. Clearly, while not all the combinations formed in this way are functional, a great many are. The net result is a great range of possible functions achieved with relative economy of means.

THE CASE OF THE 5-SECOND MESSENGER MOLECULE

In another trail of research, molecular biology and genetics are joining pharmacology to bring to light the workings of yet one more type of messenger molecule. The molecule nitric oxide fills a critical role in diverse tissues of the body, from the lining of blood vessels to the cerebellum, but its identity as a messenger of anything at all was completely unsuspected for a long time. For one thing, the compound—a single atom of nitrogen joined to one atom of oxygen—was very unstable, existing only for a matter of seconds. What could it be doing in the body?



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