UAS regulatory sequences drive expression of the 'psMEK[E203K]' photoswitchable kinase. 'psMEK[E203K]' consists of the psMEK1tight photoswitchable kinase from PMID:28232577, with an additional E203K amino acid substitution, thus it consists of Maur\MEK1 that has been mutated to contain i. two phosphomimetic mutations in the active loop (S218D and S222D), resulting in a constitutively active enzyme, ii. the E203K amino acid substitution and iii. two insertions of pdDronpa1.2 : one after the N-terminal substrate docking site (insertion after codon 60), and the other in the FG loop (insertion after codon 304). The two pdDronpa1.2 moieties reversibly cage the Maur\MEK1 active site; when the two pdDronpa1.2 domains form a dimer, they sterically hinder access to the Maur\MEK1 active site, but when the pdDronpa1.2 domains dissociate, the active site of the constitutively active Maur\MEK1 is exposed. 'psMEK[E203K]' can be reversibly switched between the two conformations in a light-regulated manner: illumination with light of wavelength 400nm favours the closed conformation (dimerized pdDronpa1.2 moieties), while light of wavelength 500nm favors the open conformation in which the Maur\MEK1 active site is exposed.