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J. Biol. Chem., Vol. 282, Issue 39, 28740-28748, September 28, 2007

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Novel Heme-based Oxygen Sensor with a Revealing Evolutionary History^*

Oleg V. Moskvin, Samuel Kaplan, Marie-Alda Gilles-Gonzalez^¶, and Mark Gomelsky¹

From the Department of Molecular Biology, University of Wyoming, Laramie, Wyoming 82071, the Department of Microbiology and Molecular Genetics, The University of Texas Medical School, Houston, Texas 77030, and the ^¶Department of Biochemistry, The University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390

To monitor fluctuations in oxygen concentration, cells use sensory proteins often containing heme cofactors. Here, we identify a new class of heme-binding oxygen sensors, reveal their unusual phylogenetic origin, and propose a sensing mode of a member of this class. We show that heme is bound noncovalently to the central region of AppA, an oxygen and light sensor from Rhodobacter sphaeroides. The addition of oxygen to ferrous AppA discoordinated the heme, and subsequent oxygen removal fully restored the heme coordination. In vitro, the extent of heme discoordination increased gradually with the rise in oxygen levels over a broad concentration range. This response correlated well with the gradual decrease in transcription of photosynthesis genes regulated by AppA and its partner repressor PpsR. We conclude that the AppA-PpsR regulatory system functions as an oxygen-dependent transcriptional rheostat. We identified a new domain embedded in the central region of AppA and designated it SCHIC for sensor containing heme instead of cobalamin. A phylogenetic analysis revealed that SCHIC domain proteins form a distinct cluster within a superfamily that includes vitamin B₁₂-binding proteins and other proteins that may bind other kinds of tetrapyrroles.

Received for publication, April 18, 2007 , and in revised form, July 19, 2007.

^* This work was supported by National Institutes of Health National Center for Research Resources (COBRE) Grant P20 RR15640 (to M. G.) and National Institutes of Health Grant GM15590 (to S. K.), National Research Initiative of the United States Department of Agriculture Cooperative State Research, Education and Extension Service, Grant 2002-35318-14039 (to M. A. G. G.), and National Science Foundation Grant 620531 (to M. A. G. G.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement"in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Molecular Biology, Dept. 3944, University of Wyoming, 1000 E. University Ave., Laramie, WY 82071. Tel.: 307-766-3522; Fax: 307-766-3875; E-mail: gomelsky{at}uwyo.edu.

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