Changes in the conserved stromal Cys of STN7 do not impair state transitions in Arabidopsis. A, Three-week-old wild-type (Col-0) and Cys mutant seedlings of Arabidopsis were subjected to state transitions. In these mutants the conserved Cys C1, C2, C3, and C4 of STN7 (see Fig. 1) were changed to Ala and are labeled as Stn7-ΔCys-1, -2, -3, -4; Stn7-ΔCys-1+2 refers to the C1C2 double mutant. Before the fluorescence measurements, seedlings were transferred to the dark for 30 min. They were then subjected to white light (state 2). After 12 min, far-red light was superimposed for 12 min (state 1). Fs levels were continuously monitored and Fm was determined by using saturating flashes as indicated with arrows (off-scale). All curves were normalized to (F−F0)/F0. B, Quantification of state transitions by the “ST, Fm” and “ST, Steps” methods (for details, see “Materials and Methods”). C, Schematic view of the STN7 kinase of Arabidopsis with the Cys residues.