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Figure 1.2 HemAT-Bs crystal structures. (A) The dimerization interface of the
unliganded HemAT-Bs homo-dimer is reported (PDB: 1OR6). The contact area is wide
and involves helices G and H in an antiparallel four-helical bundle, part of the
Z-helix, and the BC corner. (B) Superposition of HemAT-Bs unliganded (light grey)
and liganded (dark grey, PDB: 1OR4) structures. The presence of the ligand (CN ) causes
conformational modifications in the haem group and in the haem-pocket. These
changes are essential for signal transduction. The haem group and the ligand are
depicted with ball-and-stick representation.
binds one gas molecule and passes into the energetically favourable liganded
state, undergoing structural rearrangements. These structural modifications
further decrease O 2 affinity of the second molecule of the dimer. As the
organismmoves along the gradient, the O 2 concentration increases progres-
sively until it saturates the second subunit ( Zhang & Phillips, 2003 ).
2.2. Haem-based sensors with gene-regulating function
The regulation of gene expression occurs via (i) DNA direct binding, for
example, carbon monoxide oxidation activator (CooA) ( Aono, Nakajima,
Saito, & Okada, 1996; Aono, Takasaki, Unno, Kamiya, & Nakajima,
1999; He, Shelver, Kerby, & Roberts, 1996; Lanzilotta et al., 2000;
Rajeev et al., 2012; Roberts, Thorsteinsson, Kerby, Lanzilotta, & Poulos,
2001; Shelver, Kerby, He, &Roberts, 1997 ) and neuronal PAS domain pro-
tein 2 (NPAS2) ( Dioum et al., 2002; Gekakis et al., 1998; Hogenesch, Gu,
Jain, & Bradfield, 1998; Reick, Garcia, Dudley, & McKnight, 2001 );
(ii) protein-protein interaction as in transcription factors and regulators,
for example, nitrogen fixation gene expression regulator (FixL), Vv GReg,
and Cv GReg ( David et al., 1988; Freitas, Hou, & Alam, 2003; Freitas,
Saito, Hou, & Alam, 2005; Gilles-Gonzalez, Ditta, & Helinski, 1991;
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