Fluorescent Timer was used to trace the activity of the Otx -2 promoter, which is involved in Xenopus laevis development. As development proceeds to the tadpole stage, the expression is almost completely suppressed in these regions, but is strong in the mesencephalic area. Figure 3 shows a Xenopus tadpole that was microinjected at the embryonic stage with a plasmid containing the Fluorescent Timer gene under the control of the Otx -2 promoter. The fluorescent image reveals that the telencephalon of the tadpole is red, indicating that the Otx -2 promoter was once active there but is now turned off, giving the accumulated protein time to mature. In contrast, the mesencephalon and diencephalon are green, indicating that Otx -2 promoter was active in these areas at the time of photography.
Timer construct produces a red signal in the tadpole brain.
Figure 3C shows an in situ hybridization in which Otx -2 expression is strong in the tadpole mesencephalon and diencephalon. Although useful for corroborating fluorescent results, in situ hybridization is not as revealing as Fluorescent Timer because it does not indicate whether expression occurred in regions where it is currently not visible. Fluorescent Timer reveals spatial and temporal changes in a target promoter's activity during development. The mosaic fluorescent images were composed from clones of cells. Panel B. Fluorescent Timer expression driven by the Otx -2 promoter in the tadpole brain.