Cyclic-AMP is one of the most important second messengers，regulating many crucial cellular events in both prokaryotes and eukaryotes. Precise spatial and temporal control of cAMP levels by light shows great promise as a simple means of manipulating and studying numerous cell pathways and processes. The photoactivated adenylate cyclase（PAC）from the photosynthetic cyanobacterium Oscillatoria acuminata is a small homodimer eminently suitable for this task，requiring only a simple flavin as chromophore. Here we describe its structure using X-ray crystallography and crystal microspectrophotometry. Site-directed mutants show signal transduction over 30 Ångstroms across the protein with minimal conformational rearrangement. The use of the protein in living human cells is demonstrated with cAMP-dependent luciferase，showing a rapid and stable response to light over many hours and activation cycles.