Studies of theDrosophila norpA phototransduction mutant

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Abstract
The electrophysiological characteristics ofnorpA H52 , a temperature sensitive phototransduction mutant ofDrosophila melanogaster, were studied in vivo. Upon raising the environmental temperature to 33–37 °C, mutant flies exhibited time-dependent changes in photoresponses. Initial observations were losses in responsiveness at low light intensities and prolonged receptor potential waveforms. Next, reductions in response amplitudes at higher light intensities occurred, until no responses were obtained. On return to lower temperature the electrophysiological properties recovered in reverse order. Based on these observations we conclude that the primary defect ofnorpA affects the efficiency of the phototransduction process. Enhanced light exposure could offset the receptor potential changes innorpA. With the temperature sensitive mutant: (1) additional light exposure prolonged the time that responses could be observed at the higher temperature, (2) when 1-s illuminations no longer elicited responses at the higher temperature, 1-min illuminations at the same intensity temporarily restored the ability to obtain 1-s-responses, and (3) light accelerated the restoration of responses on return to lower temperature. Illumination also had an effect on non-temperature sensitivenorpA mutants, enabling the production of small photoresponses innorpA H44 , a mutant that normally does not exhibit any responses, and improving the low-light-intensity responses ofnorpA P16 . Our study indicates that the PI cycle, which is inhibited innorpA mutants (Yoshioka et al. 1985), is an important light-sensitive positive step or effector in the production of receptor potential responses.