Functions in association with RUP2 as repressor of UV-B-induced photomorphogenesis mediated by UVR8 and HY5, likely in coordination with DHU1 (PubMed:28735869). Plays a crucial negative feedback regulatory role downstream of UVR8-COP1 to inhibit UVR8 function, balance UV-B-specific responses and ensure normal plant growth. Is involved in the regulation of photoperiodic flowering and vegetative development. Interacts with UVR8 (PubMed:21041653, PubMed:22988111). Interacts directly with DHU1 (PubMed:28735869). Circadian-regulation. Expression increases during the dark phase with a peak at the beginning of the light phase and then decreases to reach the lowest expression at the end of the light phase. Induced by UV-B. No visible phenotype under normal growth conditions (PubMed:21242318). The UV-B responsiveness of the double mutant dhu1-1 rup1-1 is similar to that of the single mutants dhu1-1 and rup1-1 (PubMed:28735869). Overexpression of RUP1 confers an early flowering phenotype.