Blue- and green-absorbing visual pigments of Drosophila: Ectopic expression and physiological characterization of the R8 photoreceptor cell-specific Rh5 and Rh6 rhodopsins

Color discrimination requires the input of different photoreceptor cells th at are sensitive to different wavelengths of light. The Drosophila visual s ystem contains multiple classes of photoreceptor cells that differ in anato mical location, synaptic connections, and spectral sensitivity. The Rh5 and Rh6 opsins are expressed in nonoverlapping sets of R8 cells and are the on ly Drosophila visual pigments that remain uncharacterized. In this study, w e ectopically expressed Rh5 and Rh6 in the major class of photoreceptor cel ls (R1-R6) and show them to be biologically active in their new environment . The expression of either Rh5 or Rh6 in "blind" ninaE(17) mutant flies, wh ich lack the gene encoding the visual pigment of the R1-R6 cells, fully res cues the light response. Electrophysiological analysis showed that the maxi mal spectral sensitivity of the R1-R6 cells is shifted to 437 or 508 nm whe n Rh5 or Rh6, respectively, is expressed in these cells. These spectral sen sitivities are in excellent agreement with intracellular recordings of the R8p and R8y cells measured in Calliphora and Musca. Spectrophotometric anal yses of Rh5 and Rh6 in vivo by microspectrophotometry, and of detergent-ext racted pigments in vitro, showed that Rh5 is reversibly photoconverted to a stable metarhodopsin (lambda(max) = 494 nm), whereas Rh6 appears to be pho toconverted to a metarhodopsin (lambda(max) = 468 nm) that is less thermall y stable. Phylogenetically, Rh5 belongs to a group of short-wavelength-abso rbing invertebrate visual pigments, whereas Rh6 is related to a group of lo ng-wavelength-absorbing pigments and is the first member of this class to b e functionally characterized.