Figure 6.

Schematic presentation of the three levels in phytochrome signaling and their relations. Each level resides in an equilibrium that is affected by the other levels or external factors (denoted by arrows and k-factors). In the case of CBD–PHY-Y263F, the Pr→Pfr transition is impaired, but the α-helical tongue is favored over the β-sheet, which increases the prospects of attaining an open PSM that has its biliverdin in the Pr (ZZZ) state. Multiple arrows between the “biliverdin” and “tongue” levels illustrate multiple, partly redundant signaling routes between the levels.