Phototransduction in transgenic mice after targeted deletion of the rod transducin alpha-subunit

Retinal photoreceptors use the heterotrimeric: G protein transducin to coup le rhodopsin to a biochemical cascade that underlies the electrical photore sponse. Several isoforms of each transducin subunit are present in the reti na. Although rods and cones seem to contain distinct transducin subunits, i t is not known whether phototransduction in a given cell type depends stric tly on a single form of each subunit. To approach this question, we have de leted the gene for the rod transducin cu-subunit in mice. In hemizygous kno ckout mice, there was a small reduction in retinal transducin cr-subunit co ntent but retinal morphology and the physiology of single rods were largely normal. In homozygous knockout mice, a mild retinal degeneration occurred with age. Rod-driven components were absent from the electroretinogram, whe reas cone-driven components were retained. Every photoreceptor examined by single-cell recording failed to respond to flashes, with one exception. The solitary responsive cell was insensitive, as expected for a cone, but had a rod-like spectral sensitivity and flash response kinetics that were slow, even for rods. These results indicate that most if not ail rods use a sing le transducin type in phototransduction.