(A) Structure of caged fluorescein. R1, R2, and R3 indicate different groups in various caged fluoresceins that have been synthesized. The fluorescein ring system is locked into a nonfluorescent tautomer by virtue of attached o-nitrobenzene groups. (B) Cartoon showing uncaging strategy. The ether bonds that link the o-nitrobenzene groups to the fluorescein ring are photolabile. Irradiation with 360-nm light cleaves the ether bonds, and the fluorescein then rapidly adopts its fluorescent (xanthen-3-one) form. The black lines represent caged (nonfluorescent) oligonucleotides hybridized to RNA molecules (gray) inside the nucleus. The blue circle represents an uncaging site in which the green lines represent uncaged oligonucleotides hybridized to RNA. These molecules move out from the uncaging site over time (right side of diagram). (Modified from Politz 1999 and reprinted with permission from Elsevier © 1999.)