Evidence for indirect control of phospholipase C (PLC-beta) by retinoids in Drosophila phototransduction

Purpose: To determine how retinoids regulate the phospholipase C (PLC) gene in the Drosophila visual system. Methods: Western blotting, activity analyses and immunocytochemistry were a pplied to Drosophila reared on various diets. Results: Western blots and activity analyses showed that retinoid deprivati on decreases PLC, the product of the norpA gene, by approximately 1/3 to 1/ 2 in Drosophila. Immunocytochemistry using standard and confocal fluorescen ce microscopy confirmed the expectation that PLC is localized to the photor eceptive rhabdomeres. Rhabdomeres of flies that were retinoid deprived, or reared on other diets devoid of chromophore precursors, fluoresced brightly . These observations are consistent with earlier morphometric analyses show ing that retinoid deprivation decreases the size of rhabdomeres. In a separ ate control, rhabdomeric PLC was shown to be virtually eliminated by retino id deprivation in transgenic Drosophila where the norpA coding sequence was driven by the opsin promoter. Conclusions: PLC is decreased by retinoid deprivation. Retinoid control of PLC is indirect, as expected, since the norpA promoter is so different from the promoter for rhodopsin's gene. PLC is not eliminated by deprivation bu t decreases in proportion to the associated decrease in rhabdomere size whi ch, in turn, is caused by the opsin decrease. By contrast, opsin is control led by retinoids both translationally by chromophore availability and trans criptionally. The fact that PLC is eliminated by retinoid deprivation when opsin's promoter drives the PLC gene is important evidence substantiating r etinoid control via opsin's promoter.