Biophysical studies performed to elucidate the mechanisms how the euglenoid pbotoreceptor photoactivated adenylyl cyclase (PAC) converts light signal to cyclic AMP, the versatile second messenger, have led to the following outcomes: electron micrographic details of its tetrameric structure, spectroscopic unveiling of its two states of hydrogen bond networks, division of labor between high-sensitivity-low-speed and the opposite subunits, invention of the single-protein infrared spectroscopic method, photomanipulation of neuronal and behavioral activities of various organisms by heterologous expression of PAC, and elucidation of PAC gene localization along the <i>Euglena</i>chromosomes.