Spectral tuning of avian violet- and ultraviolet-sensitive visual pigments

The violet- and ultraviolet-sensitive visual pigments of birds belong to th e same class of pigments as the violet-sensitive (so-called blue) pigments of mammals. However, unlike the pigments from mammals and other vertebrate taxa which, depending on species, have lambda(max) values of either around 430 nm or around 370 nm, avian pigments are found with lambda(max) values s pread across this range. In this paper, we present the sequences of two pig ments isolated from Humbolt penguin and pigeon with intermediate lambda(max ) values of 403 and 409 nm, respectively. By comparing the amino acid seque nces of these pigments with the true UV pigments of budgerigar and canary a nd with chicken violet with a lambda(max) value of 420 nm, we have been abl e to identify five amino acid sites that show a pattern of substitution bet ween species that is consistent with differences in lambda(max). Each of th ese substitutions has been introduced into budgerigar cDNA acid expressed i n vitro in COS-7 cells. Only three resulted in spectral shifts in the regen erated pigment; two had relatively small effects and may account for the sp ectral shifts between penguin, pigeon, and chicken whereas one, the replace ment of Ser by Cys at site 90 in the UV pigments, produced a 35 nm shortwav e shift that could account for the spectral shift from 403 nm in penguin to around 370 nm in budgerigar and canary.