Opsin-Mediated Inhibition of Bacterioruberin Synthesis in Halophilic Archaea.

Halophilic archaea often inhabit environments with limited oxygen, and many produce ion-pumping rhodopsin complexes that allow them to maintain electrochemical gradients when aerobic respiration is inhibited. Rhodopsins require a protein, an opsin, and an organic cofactor, retinal. We previously demonstrated that in , bacterioopsin (BO), when not bound by retinal, inhibits the production of bacterioruberin, a biochemical pathway that shares intermediates with retinal biosynthesis. In this work, we used heterologous expression in a related halophilic archaeon, , to demonstrate that BO is sufficient to inhibit bacterioruberin synthesis catalyzed by the lycopene elongase (Lye) enzyme. This inhibition was observed both in liquid culture and in a novel colorimetric assay to quantify bacterioruberin abundance based on the colony color. Addition of retinal to convert BO to the bacteriorhodopsin complex resulted in a partial rescue of bacterioruberin production. To explore if this regulatory mechanism occurs in other organisms, we expressed a Lye homolog and an opsin from in cruxopsin-3 expression inhibited bacterioruberin synthesis catalyzed by Lye but had no effect when bacterioruberin synthesis was catalyzed by or Lye. Conversely, BO did not inhibit Lye activity. Together, our data suggest that opsin-mediated inhibition of Lye is potentially widespread and represents an elegant regulatory mechanism that allows organisms to efficiently utilize ion-pumping rhodopsins obtained through lateral gene transfer. Many enzymes are complexes of proteins and nonprotein organic molecules called cofactors. To ensure efficient formation of functional complexes, organisms must regulate the production of proteins and cofactors. To study this regulation, we used bacteriorhodopsin from the archaeon Bacteriorhodopsin consists of the bacterioopsin protein and a retinal cofactor. In this article, we further characterize a novel regulatory mechanism in which bacterioopsin promotes retinal production by inhibiting a reaction that consumes lycopene, a retinal precursor. By expressing genes in a different organism, , we demonstrated that bacterioopsin alone is sufficient for this inhibition. We also found that an opsin from has inhibitory activity, suggesting that this regulatory mechanism might be found in other organisms.