Keffer, J. L.Hahn, M. W.Maresca, Julia Anne2015-11-172015-11-17Copyright2015-06-08Keffer,J.L.; Hahn,M.W.; Maresca,J.A. ,Characterization of an Unconventional Rhodopsin from the Freshwater Actinobacterium Rhodoluna lacicola. Journal of Bacteriology, 2015, 197, 16, 2704-27120021-9193http://udspace.udel.edu/handle/19716/17226Accepted manuscript posted online.Rhodopsin-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 actinobacterium, Rhodoluna lacicola encodes a rhodopsin of the uncharacterized actinorhodopsin family. We hypothesized that actinorhodopsin was a light-activated proton pump, and confirmed this by heterologously expressing R. lacicola actinorhodopsin in retinal-producing Escherichia coli. However, cultures of R. lacicola did not pump protons, even though actinorhodopsin mRNA and protein were both detected. Proton pumping in R. lacicola was induced by providing exogenous retinal, suggesting that the cells lacked the retinal cofactor. We used HPLC and oxidation of accessory pigments to confirm that R. lacicola does 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.en-USArticle is made available in accordance with the University of Delaware Faculty Policy on Open Access and the publisher's policy.Articledoi:10.1128/JB.00386-15