Biology Reference
In-Depth Information
(
Carrica Mdel, Fernandez, Mart
ยด
, Paris, & Goldbaum, 2012
). NtrY
(BAB1_1139) is the histidine kinase in the NtrY/NtrX two-component sys-
tem found in
Brucella
spp. that are facultative intracellular gram-negative
bacteria that belong to the
a
2 proteobacteria group. The expression of all
the denitrification enzymes is increased in low O
2
tension in these bacteria,
which is partly triggered by the NtrY/NtrX two-component system (
Al
Dahouk et al., 2009; Roop & Caswell, 2012
). NtrY from
B. abortus
is a
homologue to NtrY that is involved in nitrogen metabolism and/or fixation
in
Azorhizobium caulinodans
and
Azospirillum brasilense
(
Ishida et al., 2002;
Pawlowski, Klosse, & de Bruijn, 1991
), which contains predicted trans-
membrane regions at the N-terminus followed by a HAMP, PAS, and his-
tidine kinase domains (
Carrica Mdel et al., 2012
). The PAS domain binds a
haem, which shows the Soret,
a
, and
b
bands at 408, 558, and 533 nm, and
423, 555, and 527 nm in the ferric and ferrous states, respectively (
Carrica
Mdel et al., 2012
). While a stable O
2
-bound NtrY does not form due to an
autoxidation to produce the ferric form, NO and CO can bind to the ferrous
haem in NtrY to form a 5-coordinate nitrosyl haem and 6-coordinate
CO-bound haem, respectively (
Carrica Mdel et al., 2012
). Ferrous NtrY
possesses much higher autokinase activity than ferric NtrY. The autokinase
activity of CO-bound and NO-bound NtrY is similar to that of ferrous
NtrY (
Carrica Mdel et al., 2012
), though the coordination structures of
the CO- and NO-bound haems are different from each other. The reason
of this unique property is not obvious at present. Based on these results, it is
proposed that NtrY acts as a redox sensor, not a gas sensor (
Carrica Mdel
et al., 2012
). Alignments of amino acid sequences among NtrY and other
haem-containing PAS domains reveal that NtrY does not show a conserved
His as the proximal haem ligand within its PAS domain, which suggests that
the haem binds to the PAS domain in NtrY in a different manner to that
described for other haem-containing PAS domains. However, more
detailed characterisation with spectroscopic and structural methods is
required to prove this hypothesis with identification of the coordination
structure of the haem in NtrY.
4.3. DosS and DosT using a haem-containing GAF domain
as a sensor module
4.3.1 Regulation of kinase activities of DosS and DosT from
Mycobacterium tuberculosis
GAF domains represent one of the largest and most widespread domain
families, which provide a variety of functions including binding of small
