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Science  05 Mar 2010:
Vol. 327, Issue 5970, pp. 1180-1181
DOI: 10.1126/science.327.5970.1180-d

ATP synthase (green) in the outer membrane.

CREDIT: KÜPER ET AL., PROC. NATL. ACAD. SCI. U.S.A. 107, 3152 (2010)

Ion-motive ATP synthases are the primary producers of ATP in mitochondria, chloroplasts, bacteria, and archaea. They convert a transmembrane gradient of ions (H+ or Na+) into cytoplasmic ATP, and hence can only operate—in concert with the ion gradient–generating enzymes—in membranes that are relatively impermeant to H+ or Na+. In the cases of mitochondria, chloroplasts, and Gram-negative bacteria, this is the inner membrane; the outer membrane is too leaky to support the establishment of ion gradients. Küper et al. report that in the archaeon Ignicoccus hospitalis, both the ATP synthase and the H2:sulfur oxidoreductase are located in the outer membrane. Ignicoccus obtains energy by reducing elemental sulfur with molecular hydrogen, and its inner membrane encloses the customary complement of DNA and ribosomes. Unlike the aforementioned organelles and cells, however, the intermembrane compartment is as large as the inner membrane–bounded cytoplasm. This sizable periplasmic space likely supports ATP-consuming metabolic reactions such as the fixation of CO2, the sole carbon source for Ignicoccus. The authors point out that in an earlier age, engulfed bacteria might have found this environment hospitable.

Proc. Natl. Acad. Sci. U.S.A. 107, 3152 (2010).

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