Fig. 2.

ERGs recorded from transgenic flies expressing Rh5 and Rh6. A, Responses to different wavelengths are arranged in columns, and responses recorded from specific genetic backgrounds are arranged inrows. Control y w flies (toprow of traces), which express the Rh1 opsin in the R1–R6 photoreceptor cells, have a robust response to light at both 430 and 520 nm. The depolarization is preceded and followed by on- and off-transients, respectively, which originate in the lamina and reflect the activation of the R1–R6 cells (Heisenberg, 1971; Heisenberg and Wolf, 1984; Laughlin, 1989). The mutant host strain that lacks the Rh1 opsin, y w; ninaE¹⁷ (secondrow of traces from thetop), has a dramatically reduced receptor potential and lacks both on- and off-transients. The residual response is derived from the R7 and R8 photoreceptor cells that are unaffected by theninaE¹⁷ mutation. The transgenic flies that express Rh5 or Rh6, y w; ninaE¹⁷P[Rh1 + 5, y⁺](thirdrow of traces from the top) and y w; ninaE¹⁷P[Rh1 + 6, y⁺](fourthrow oftraces from the top), display robust, wild-type photoresponses at either wavelength of light, demonstrating that both Rh5 and Rh6 are functional when expressed in the R1–R6 photoreceptor cells. Response amplitudes are not directly comparable between strains because of differences in the transgene expression levels. All of the strains were stimulated with the same intensity of light, which was ∼1.09 μW/cm² at 430 nm and 0.9 μW/cm² at 520 nm. B,When a control fly (y w; toptrace) is stimulated with intense light at 570 nm, there are a robust depolarization and immediate repolarization at the end of the stimulus. A PDA is induced when the fly is stimulated with light at 470 nm (thus producing a large amount of activated M-form). The depolarization is maintained after the cessation of the stimulus, and the photoreceptor cells are relatively inactivated to further stimuli. When the fly is again stimulated with 570 nm light, the PDA is terminated by the photoconversion of the M-form back to the R-form. Transgenic flies expressing Rh5 (y w; ninaE¹⁷ P[Rh1 + 5, y⁺]; secondtrace from the top) can undergo a PDA when stimulated with light at 430 nm, and this can be terminated by stimulation with light at 520 nm. Transgenic flies expressing Rh6 (y w; ninaE¹⁷ P[Rh1 + 6, y⁺]; thirdtrace from the top) do not show a PDA when stimulated with 550 nm light. This wavelength would be expected to produce maximal conversion of Rh6 from the R- to the M-form with minimal photoconversion back to the R-form (see Fig. 4, for Rh6 extract difference spectra). Stimulation of Rh6 transgenic flies at 350, 430, 470, 520, and 570 nm was also insufficient to induce a PDA (data not shown). Light intensity was unattenuated in these experiments and was ∼0.5 mW/cm² at each of the wavelengths tested. Thebottomrow in A and therowbelow each trace inB show the stimulus, with the time and voltage scales indicated on the bottomleft of eachpanel.