Analysis of the blind Drosophila mutant ninaB identifies the gene encodingthe key enzyme for vitamin A formation in vivo

Visual pigments (rhodopsins) are composed of a chromophore (vitamin A deriv ative) bound to a protein moiety embedded in the retinal membranes. Animals cannot synthesize the visual chromophore de novo but rely on the uptake of carotenoids, from which vitamin A is formed enzymatically by oxidative cle avage. Despite its importance, the enzyme catalyzing the key step in vitami n A formation resisted molecular analyses until recently, when the successf ul cloning of a cDNA encoding an enzyme with beta,beta -carotene-15,15'-dio xygenase activity from Drosophila was reported. To prove its identity with the key enzyme for vitamin A formation in vivo, we analyzed the blind Droso phila mutant ninaB. In two independent ninaB alleles, we found mutations in the gene encoding the beta,beta -carotene-15,15'-dioxygenase. These mutati ons lead to a defect in vitamin A formation and are responsible for blindne ss of these flies.