Fig. 4. Photoactivation, photoconversion and photoswitching mechanisms for optical highlighter FPs. (A) Photoactivation of PA-GFP (illustrated) and PS-CFP2 is believed to occur due to decarboxylation of Glu222 followed by conversion of the chromophore from a neutral to anionic state. (B) Green-to-red photoconversion for Kaede, KikGR, Dendra2 and Eos, all of which contain the HYG chromophore, occurs when the FP is illuminated with ultraviolet or violet radiation to induce cleavage between the amide nitrogen and -carbon atoms in the His62 residue leading to subsequent formation of a conjugated dual imidazole ring system. (C) Photoswitching of Dronpa involves cis-trans photoisomerization induced by alternating radiation between 405 nm and 488 nm. A similar isomerization mechanism is suggested to operate in mTFP0.7 and KFP1.