Screening random mutations of the red fluorescent protein drFP583 from tropical coral, researchers at the National Institutes of Health, Stanford University School of Medicine, the Institute of Bioorganic Chemistry at the Russian Academy of Science, and Palo Alto, Calif.-based BD Biosciences-CLONTECH made an unusual discovery.1 After fluorescing green for about three hours, a mutant protein called E5 matures and begins to fluoresce red; thus, E5 acts like a stopwatch, telling researchers when the gene is turned on as well as if and when it's turned off.

The authors tested the novel fluorescent protein in HEK 293 mammalian cells using a tetracycline-inducible expression system, in Caenorhabditis elegans embryos using heat shock-regulated expression, and in Xenopus laevis to test Otx-2-regulated expression. In all cases when promoters were recently turned on, the cells fluoresced green. When they stopped expressing, the cells fluoresced red after three hours. And in cases of continuous...

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