Each Drosophila opsin binds the carotenoid chromophore 11-cis-3-hydroxyretinal via a lysine. This lysine is conserved in almost all opsins, only a few opsins have lost it during evolution. Opsins without it are not light sensitive. In particular, the Drosophila opsins Rh1, Rh4, and Rh7 function not only as photoreceptors, but also as chemoreceptors for aristolochic acid. These opsins still have the lysine like other opsins. However, if it is replaced by an arginine in Rh1, then Rh1 loses light sensitivity but still responds to aristolochic acid. Thus, the lysine is not needed for Rh1 to function as chemoreceptor.
Each Drosophila opsin binds the carotenoid chromophore 11-cis-3-hydroxyretinal via a lysine. This lysine is conserved in almost all opsins, only a few opsins have lost it during evolution. Opsins without it are not light sensitive In particular, the Drosophila opsins Rh1, Rh4, and Rh7 function not only as photoreceptors, but also as chemoreceptors for aristolochic acid These opsins still have the lysine like other opsins. However, if it is replaced by an arginine in Rh1, then Rh1 loses light sensitivity but still responds to aristolochic acid Thus, the lysine is not needed for Rh1 to function as chemoreceptor.