Protein kinase that acts as a blue light photoreceptor in a signal-transduction pathway for phototropic responses. Regulates a wide range of physiological activities in plants that maximize the efficiency of photosynthesis, such as chloroplast relocations, stomata opening, and leaf expansion (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Binds 2 FMN per subunit. Exhibits a smaller absorbance peak at 350 nm. The broad fluorescence emission spectrum peaks at 490 nm. Homodimer. The PAS (PER-ARNT-SIM) domains are required for the binding of FMN chromophores. Autophosphorylated in response to blue light irradiation. 2 molecules of FMN bind covalently to cysteines after exposure to blue light and are reversed in the dark. Undergoes a photocycle characterized by fluorescence and absorption changes induced by blue light. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.