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Figure 7.7 Interaction of TrrA with ThkA. (A) Close-up view of the interaction of TrrA
with ThkA. (B) The structure of TrrA monomer. The regions interacting with ThkA, which
are evaluated by NMR, are shown in blue and orange. Chemical shift perturbations
(blue) and disappearance of the cross-peak (orange) are observed upon addition of
an excess amount of the (DHpþCA) domain of ThkA to TrrA. Reproduced with permission
from Yamada et al. (2009) .
a 7 helix ( Yamada et al., 2009 ), indicating that phosphorylation of His547
cannot take place in this structure. Marina, Waldburger, and Hendrickson
(2005) propose that the CA domain of histidine kinase should move signif-
icantly towards the phosphoacceptor His in the DHp domain. In the ThkA/
TrrA complex, the interdomain b sheet with the PAS and CA domains will
prevent the CA domain from moving towards the DHp domain ( Yamada
et al., 2009 ). Thus, the structure of ThkA might be an inactive form for
autokinase activity, which will correspond to the O 2 -bound form of FixL.
If O 2 dissociation weakens the interdomain interaction by inducing a con-
formational change in the G b , the CA domain would be permitted to freely
move for autokinase reaction ( Yamada et al., 2009 ).
4.2. A haem-based redox sensor in NtrY/NtrX
two-component system
Genome sequence analyses reveal that Brucella abortus genome encodes
10 proteins with predicted PAS domains, 8 are associated with histidine
kinase domains and 2 are associated with GGDEF/EAL domains, among
which 5 proteins (BAB1_0640, 1139, 1621, 2101, and 0220) are predicted
to contain a haem as a prosthetic group though it is not proved experimentally
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