Characterization of an Unconventional Rhodopsin from the Freshwater Actinobacterium Rhodoluna lacicola
ABSTRACTRhodopsin-encoding microorganisms are common in many environments. However, knowing that rhodopsin genes are present provides little insight into how the host cells utilize light. The genome of the freshwater actinobacteriumRhodoluna lacicolaencodes a rhodopsin of the uncharacterized actinorhodopsin family. We hypothesized that actinorhodopsin was a light-activated proton pump and confirmed this by heterologously expressingR. lacicolaactinorhodopsin in retinal-producingEscherichia coli. However, cultures ofR. lacicoladid not pump protons, even though actinorhodopsin mRNA and protein were both detected. Proton pumping inR. lacicolawas induced by providing exogenous retinal, suggesting that the cells lacked the retinal cofactor. We used high-performance liquid chromatography (HPLC) and oxidation of accessory pigments to confirm thatR. lacicoladoes not synthesize retinal. These results suggest that in some organisms, the actinorhodopsin gene is constitutively expressed, but rhodopsin-based light capture may require cofactors obtained from the environment.IMPORTANCEUp to 70% of microbial genomes in some environments are predicted to encode rhodopsins. Because most microbial rhodopsins are light-activated proton pumps, the prevalence of this gene suggests that in some environments, most microorganisms respond to or utilize light energy. Actinorhodopsins were discovered in an analysis of freshwater metagenomic data and subsequently identified in freshwater actinobacterial cultures. We hypothesized that actinorhodopsin from the freshwater actinobacteriumRhodoluna lacicolawas a light-activated proton pump and confirmed this by expressing actinorhodopsin in retinal-producingEscherichia coli. Proton pumping inR. lacicolawas induced only after both light and retinal were provided, suggesting that the cells lacked the retinal cofactor. These results indicate that photoheterotrophy in this organism and others may require cofactors obtained from the environment.