RESEARCH HIGHLIGHTS HORIZONTAL GENE TRANSFER Eukaryotes under a new light Proteorhodopsins are light-driven In bacteria, proteorhodopsins proton pumps that harvest energy generate a proton gradient across the from light to help bacteria and plasma membrane, but eukaryotic archaea grow and survive when cells possess complex membranous nutrients are scarce. Now, Slamovits systems and, therefore, there are and colleagues show that bacteria- many compartments in which a derived proteorhodopsins are present gradient can be established. To in some marine protists. determine the subcellular location of During a genomic analysis of the O. marina proteorhodopsins, an various marine dinoflagellates, antibody was developed against their the authors sequenced >18,000 deduced carboxyl termini. Using expressed sequence tags (ESTs) immunofluorescence microscopy, from Oxyrrhis marina, a predatory the authors showed that the proteins microorganism that is abundant in were distributed unevenly within the shallow waters, and found several cytosol, in an unidentified fraction of sequences encoding proteins that the endomembrane system. were highly similar to bacterial These results indicate that the proteorhodopsins. Interestingly, proteins identified in O. marina similar ESTs were also obtained are proteorhodopsins of bacterial from two other dinoflagellates, origin, but their function remains Pyrocystis lunula and Alexandrium unknown. The authors hypothesize tamarense. The conservation of Colourized scanning electron micrograph of the that the proteins may generate a dinoflagellate Oxyrrhis marina. Image courtesy of these proteorhodopsin genes among S. Breglia, University of British Columbia, proton gradient in endomembrane distantly related dinoflagellates Vancouver, Canada. compartments to force the reverse suggests that the genes may have functioning of vacuolar ATPases or been horizontally transferred from O. marina, which may explain the pyrophosphatases (which usually use marine bacteria around the time bright pink colour of the microorgan- ATP or pyrophosphate to produce of origin of the dinoflagellates. ism, as proteorhodopsins typically a proton gradient), leading to light- Moreover, the O. marina ESTs exhibit a red colour. The O. marina dependent synthesis of energy-rich included transcripts encoding a proteorhodopsins probably possess ATP or pyrophosphate. Alternatively, protein that is highly similar to seven transmembrane domains the proteorhodopsins could replace bacterial carotenoid dioxygenases as well as the key residues that are or supplement the normal function and therefore may have a role in the characteristic of the proteorhodopsin of these vacuolar enzymes by using synthesis of the carotenoid retinal, family, as indicated by alignment light to acidify digestive vacuoles. a prosthetic group found in proteo­ with bacterial proteorhodopsins Cesar Sanchez bacteria-derived rhodopsins. The dinoflagellate may and secondary-structure prediction. proteorhodopsins have acquired this enzyme either Furthermore, the presence of a par- ORIGINAL RESEARCH PAPER Slamovits, C. H. horizontally from bacteria or ticular residue (Leu109) suggests that et al. A bacterial proteorhodopsin proton pump in are present in marine eukaryotes. Nature Commun. 2, 183 (2011) vertically from an ancestral plastid. the O. marina proteins are tuned to FURTHER READING Fuhrman. J. A., some marine The proteorhodopsin sequences maximally absorb green light, which Schwalbach, M. S. & Stingl, U. Proteorhodopsins: protists. an array of physiological roles? Nature Rev. were the most highly represented is consistent with the ecology of this Microbiol. 6, 488–494 (2008) nuclear genes in the EST survey of organism. NATURE REVIEWS | MICROBIOLOGY VOLUME 9 | APRIL 2011 © 2011 Macmillan Publishers Limited. All rights reserved.