In mammalian rods and cones, light activation of the visual pigments leads to release of the chromophore, which is then recycled through a multistep enzymatic pathway, referred to as the visual or retinoid cycle. In invertebrates such as Drosophila, a visual cycle was thought not to exist since the rhodopsins are bistable photopigments, which consist of a chromophore that normally stays bound to the opsin following light activation. Nevertheless, we recently described a visual cycle in Drosophila that serves to recycle the free chromophore that is released following light-induced internalization of rhodopsin, and a retinol dehydrogenase (RDH) that catalyzes the first step of the pathway. Here, we describe the identification of a putative RDH, referred to as RDHB (retinol dehydrogenase B), which functions in thevisualcycleandindenovosynthesisofthechromophore.RDHBwasexpressedintheretinalpigmentcells(RPCs),whereitpromotedthefinal enzymatic reaction necessary for the production of the chromophore. Mutation of rdhB caused moderate light-dependent degeneration of the phototransducing compartment of the photoreceptor cells-the rhabdomeres, reminiscent of the effects of mutations in some human RDH genes. Since the first and last steps in the visual cycle take place in the RPCs, it appears that these cells are the sites of action for this entire enzymatic pathway in Drosophila.
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