HY5 stability and activity in Arabidopsis is regulated by phosphorylation in its COP1 binding domain

Arabidopsis HY5 is a bZIP transcription factor that promotes photomorphogen esis. Previous studies suggested that COP1, a negative regulator of photomo rphogenesis, directly interacts with nuclear HY5 and targets it for proteas ome-mediated degradation. Light negatively regulates the nuclear level of C OP1 and thus permits HY5 accumulation. Here we report that HY5 abundance pe aks in early seedling development, consistent with its role in promoting ph oto-morphogenesis. HY5 acts exclusively within a complex and exists in two isoforms, resulting from phosphorylation within its COP1 binding domain by a light-regulated kinase activity, Unphosphorylated HY5 shows stronger inte raction with COP1, is the preferred substrate for degradation, has higher a ffinity to target promoters and is physiologically more active than the pho sphorylated version. Therefore, HY5 phosphorylation provides an added level of light-mediated regulation of HY5 stability and activity besides nuclear COP1 levels. Regulated HY5 phosphorylation not only provides abundant and physiologically more active unphosphorylated HY5 in the light, but also hel ps to maintain a small pool of less active phosphorylated HY5 in the dark, which could be essential for a rapid initial response during dark-to-light transition.