Moskvin Oleg V, Kaplan Samuel, Gilles-Gonzalez Marie-Alda, Gomelsky Mark
Department of Molecular Biology, University of Wyoming, Laramie, Wyoming 82071.
Department of Microbiology and Molecular Genetics, The University of Texas Medical School, Houston, Texas 77030.
J Biol Chem. 2007 Sep 28;282(39):28740-28748. doi: 10.1074/jbc.M703261200. Epub 2007 Jul 27.
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 B(12)-binding proteins and other proteins that may bind other kinds of tetrapyrroles.
为了监测氧气浓度的波动，细胞会使用通常含有血红素辅因子的传感蛋白。在此，我们鉴定出一类新的血红素结合氧传感器，揭示了它们不同寻常的系统发育起源，并提出了该类成员的传感模式。我们发现血红素以非共价方式结合到球形红杆菌的氧和光传感器AppA的中心区域。向亚铁态的AppA中加入氧气会使血红素配位解离，随后去除氧气则能使血红素配位完全恢复。在体外，在较宽的浓度范围内，随着氧气水平的升高，血红素配位解离的程度逐渐增加。这种反应与受AppA及其伙伴阻遏蛋白PpsR调控的光合作用基因转录的逐渐减少密切相关。我们得出结论，AppA - PpsR调控系统作为一种依赖氧气的转录变阻器发挥作用。我们在AppA的中心区域鉴定出一个新结构域，并将其命名为SCHIC，即含血红素而非钴胺素的传感器。系统发育分析表明，SCHIC结构域蛋白在一个超家族中形成一个独特的簇，该超家族包括维生素B12结合蛋白和其他可能结合其他种类四吡咯的蛋白。
