"Fluorescent timer": Protein that changes color with time

We generated a mutant of the red fluorescent protein drFP583. The mutant (E 5) changes its fluorescence from green to red over time. The rate of color conversion is independent of protein concentration and therefore can be use d to trace time-dependent expression. We used in vivo labeling with E5 to m easure 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 d ownregulation of target promoters on the whole-organism scale.