Characterization of the Drosophila ninaB And ninaD Genes Involved in Rhodopsin Chromophore Biosynthesis

In Drosophila, vitamin A is only required to provide the chromophore of the visual pigment rhodopsin. Drosophila nina mutants are characterized by reduced rhodopsin levels in photoreceptors and altered electroretinograms. In the case of ninaB and ninaD, the mutant phenotype is rescued by dietary retinal, showing the defective rhodopsin production is due to failure to generate the chromophore of rhodopsin. The ninaB gene encodes a beta-carotene-15,15'-oxygenase responsible for the centric cleavage of beta-carotene to yield the retinal chromophore. The ninaD gene encodes a scavenger receptor mediating the cellular uptake of carotenoids. Despite their importance to the synthesis of the chromophore of rhodopsin, genetic mosaic analysis showed that ninaB and ninaD are not required in the retina, and exclusive retinal expression of either gene or both genes simultaneously does not support rhodopsin biogenesis. In contrast, nonretinal neuronal expression of ninaB and ninaD allows for rhodopsin biogenesis. To determine the cell types responsible for ninaB activity, I constructed and analyzed a transgenic fly strain carrying ninaB promoter driving GAL4 (pninaB-GAL4). The results showed that ninaB is expressed in the adult fly optic lobes and central brain. The expression profile of ninaD was investigated by construction and analysis of a transgenic fly strain carrying the ninaD promoter driving a fusion gene of ninaD and GFP (pDninaD-GFP). Immunohistochemical analysis verified that ninaD is expressed in the adult fly optic lobe and brain. The localization data obtained from both transgenic flies was confirmed by in situ hybridization analysis. Developmental RT-PCR showed that ninaB is restricted to adult fly head, while ninaD is expressed in both the head and body, and at all developmental stages. These results support a sequential model for NinaB and NinaD action in vitamin A metabolism. NinaD is the membrane receptor required for uptake of dietary carotenoids throughout the fly life cycle. Beta-carotene is transported to brain tissue and optic lobe, and cleaved by the NinaB enzyme to produce retinal chromophore. The retinal is then transported to photoreceptors for rhodopsin biogenesis.