The mammalian retina is known to contain two distinct
transducins that interact with their respective rod and
cone pigments. However, there are no reports of a nonmammalian
species having two distinct transducins. In the present
study, we report the cloning and cellular localization
of two transducin α subunits (Gαt) from the tiger
salamander. Through degenerate polymerase chain reaction
(PCR) and subsequent screening of a salamander retina cDNA
library, we have identified two forms of Gαt. When
compared to existing sequences in GenBank, the cloned subunits
showed high similarity to rod and cone transducins. The
salamander Gαt-1 has 91.2–93.7% amino acid sequence
identity to mammalian rod Gαt subunits and 79.7–80.9%
to mammalian cone Gαts. The salamander Gαt-2 has
86.2–87.9% sequence identity to mammalian cone Gαts
and 78.9–80.9% to mammalian rod Gαts at the amino
acid level. The Gαt-1 cDNA encodes 350 amino acids
while the Gαt-2 cDNA encodes 354 residues, which is
typical for rod and cone Gαts, respectively, and we
thus identified the Gαt-1 as rod and Gαt-2 as cone
Gαt. Sequences identified as effector binding sites
and GTPase activity regions are highly conserved between
the two subunits. Genomic Southern blot analysis showed
that rod and cone Gαt subunits are both encoded by
single-copy genes. Northern blot analysis identified retina-specific
transcripts of 3.0 kb for rod Gαt and 2.6 kb for cone
Gαt. Immunohistochemistry in the flat-mounted salamander
retina demonstrated that rod Gαt is localized to rods,
predominantly in the outer segments; similarly, cone Gαt
is localized to cone outer segments. The results confirm
that the two sequences encode rod and cone transducins
and demonstrate that this lower vertebrate contains two
distinct transducins that are localized specifically to
rod and cone photoreceptors.