Binding of transducin (Gt) to activated rhodopsin (MII) promotes the release of GDP from the Gt alpha subunit enabling a GTP (present in a higher concentration than GDP) to take its place. With GTP bound, Gt alpha (GNAT1-GTP) dissociates from the Gt beta:gamma subunits (GNB1:GNGT1) and from MII. MII is then available to bind and activate additional transducins. Transducin activation is the first amplifying step in visual transduction. Many findings came from bovine experiments (Pugh & Lamb 1993, Fung & Stryer 1980, Fung et al. 1981, Hofmann 1985).