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, opsin, and an organic cofactor, retinal. We have previously demonstrated that, in Halobacterium salinarum, 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 use heterologous expression in a related halophilic Archaeon, Haloferax volcanii, to demonstrate that BO is sufficient to inhibit bacterioruberin synthesis catalyzed by the H. salinarum lycopene elongase (Lye) enzyme. This inhibition was observed both in liquid cultures 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 Haloarcula vallismortis in H. volcanii. H. vallismortis cruxopsin expression inhibited bacterioruberin synthesis catalyzed by H. vallismortis Lye, but had no effect when bacterioruberin synthesis was 33 catalyzed by H. salinarum or H. volcanii Lye. Conversely, H. salinarum BO did not inhibit 34 H. vallismortis Lye activity. Together, our data suggest that opsin-mediated inhibition of 35 Lye is potentially widespread and represents an elegant regulatory mechanism that 36 allows organisms to efficiently utilize ion-pumping rhodopsins obtained through lateral 37 gene transfer.