A G protein that acts as a second messenger in the response to light by vertebrate photoreceptors. When a photon of light strikes a molecule of rhodopsin (the visual pigment in the rod cells of the retina of the eye), it activates transducin. This sets in train a cascade of events within the rod cell that amplifies the signal and evokes a response. The transducin activates numerous molecules of phosphodiesterase, which hydrolyses the signalling molecule cyclic GMP (cGMP). The resultant lowered intracellular concentration of cGMP causes closure of sodium channels in the rod-cell membrane that carry the ‘dark current’—inwardly flowing sodium ions; however, outflow of potassium ions persists, the rod cell hyperpolarizes (its internal electric potential becomes more negative), and its release of neurotransmitter to the relay neuron is reduced. When the light stimulus ceases, cGMP is regenerated by guanylate cyclase, the sodium channels reopen, transducin is deactivated, and neurotransmitter release returns to resting levels. Hence, the transducin pathway can produce a response to the smallest possible light stimulus—a single photon—a measure of its exquisite sensitivity.