Mediates phytochrome (phyA and phyB)-controlled seedling deetiolation responses such as hypocotyl elongation in response to red and far-red light (PubMed:16709197, PubMed:16384903). Required for light-induced expression of LHCB3 and CHALCONE SYNTHASE (CHS) (PubMed:16384903). Regulates negatively CONSTANS (CO) and FLOWERING LOCUS T (FT) expression and photoperiodic flowering (PubMed:16709197). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Levels are following a circadian rhythm oscillated under day/night cycles with higher levels during the night. Photomorphogenic mutant insensitive to red and far-red light leading to long hypocotyls. Flowers early particularly under long days (PubMed:16709197, PubMed:16384903). Also impaired in phytochrome-mediated end-of-day far-red light response, cotyledon expansion, far-red light block of greening, and light-induced expression of LHCB3 and CHALCONE SYNTHASE (CHS) (PubMed:16384903). Enhanced expression of CONSTANS (CO) and FLOWERING LOCUS T (FT) under long days and short days (PubMed:16709197).