Ultraviolet-B (UV-B) radiation is an integral part of sunlight and can be a stress factor for living organisms. Plants are unable to escape their environment and have evolved mechanisms that allow them to precisely monitor light conditions to adapt their growth. UVR8 is a UV-B photoreceptor that exists as a homodimer that monomerizes upon UV-B absorption. UVR8 then interacts with the E3 ligase COP1, a central repressor of light-induced development. UVR8 inhibits COP1 activity, allowing the accumulation of COP1 substrates, such as HY5, a transcriptional regulator controlling the expression of light-induced genes, including RUP1/RUP2. RUP1/RUP2 interact with UVR8 and promote UVR8 redimerization back to its inactive state. This work advances our knowledge regarding several steps of the UVR8 photocycle that are not well understood, including the mechanism of UVR8 redimerization, the link between COP1 inactivation by UVR8 and activation of HY5, and the precise mechanism how HY5 induces transcription.