Fig. 9. Model of two structurally separable PHYB actions for inhibiting PIF3 stability and transactivation activity.

Photoactivation of the N-terminal photosensory module induces the binding of PHYB’s light-sensing knot to PIF3’s APB, thereby masking the transactivation activity of the adjacent PIF3 AD. In parallel, the C-terminal output module, which mediates photobody formation, interacts weakly with PIF3 in a light-independent manner to promote PIF3 degradation. The latter action requires the dimerization of the output module via HKRD. The orange lines represent the interactions between PHYB and PIF3. Binding of PHYA to APA also inhibits the activity of the PIF3 AD. The schematic of PHYB domain structure is modified from a model by Burgie and Viestra¹².