School of Medicine, Stanford University, Stanford, CA 94305, USA.
Institute of Bioorganic Chemistry, Russian Academy of Science, 117871 Moscow, Russia.
Center for Molecular Modeling, Center for Information Technology, NIH, Building 12A, Bethesda, MD 20892, USA.
Clontech Laboratories, 1020 East Meadow Circle, Palo Alto, CA 94303, USA.
We generated a mutant of the red fluorescent protein drFP583. The mutant (E5) changes its fluorescence from green to red over time. The rate of color conversion is independent of protein concentration and therefore can be used to trace time-dependent expression. We used in vivo labeling with E5 to measure expression from the heat shock-dependent promoter in Caenorhabditis elegans and from the Otx-2 promoter in developing Xenopus embryos. Thus, E5 is a "fluorescent timer" that can be used to monitor both activation and down-regulation of target promoters on the whole-organism scale.