A visual pigment expressed in both rod and cone photoreceptors

Rods and cones contain closely related but distinct G protein-coupled recep tors, opsins, which have diverged to meet the differing requirements of nig ht and day vision. Here, we provide evidence for an exception to that rule. Results from immunohistochemistry, spectrophotometry, and single-cell RT-P CR demonstrate that, in the tiger salamander, the green rods and blue-sensi tive cones contain the same opsin. In contrast, the two cells express disti nct G protein transducin a subunits: rod a transducin in green rods and con e a transducin in blue-sensitive cones. The different transducins do not ap pear to markedly affect photon sensitivity or response kinetics in the gree n rod and blue-sensitive cone. This suggests that neither the cell topology or the transducin is sufficient to differentiate the rod and the cone resp onse.