Protein kinase involved in blue light responses (e.g. hypocotyl elongation and flowering) by phosphorylating CRY2 to reduce its stability. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Translocates mostly in nucleus upon blue light treatment, especially in nuclear bodies. Expressed in seedlings, stems, leaves and flowers. Induced by blue light. Slightly autophosphorylated. Hypersensitivity to blue light (BL) leading to shortened hypocotyls in BL. Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. Casein kinase I subfamily.