Biology Reference
In-Depth Information
M. tuberculosis
carries both a TrHbI (HbN) and a TrHbII (HbO) (
Milani
et al., 2005
). It is worth noting that HbN efficiently protects
M. tuberculosis
from nitrosative damage, contributing to its survival in the host macrophage
(
Bidon-Chanal et al., 2006
), whereas HbO may have an oxidation/reduc-
tion function because it has peroxidase activity with formation of ferryl
intermediates (
Ouellet et al., 2007
). On the other hand,
Mycobacterium leprae
only displays HbO, which is capable of protecting against NO as well as
oxidative stress (
Ascenzi, De Marinis, Coletta, & Visca, 2008
).
B. subtilis
(
Ouellet et al., 2007
) and
Thermobifida fusca
(
Bonamore et al., 2005
) encode
both a TrHbII and an FHb. To our knowledge,
Ph
TAC125 is the first
example of coexistence of genes encoding a FHb and three TrHbs (see
Section 6.2.2
).
A transcriptional analysis of the
PSHAa0030
,
PSHAa0458
and
PSHAa2217
genes encoding the TrHbs and the FHb-encoding gene was
carried out on
Ph
TAC125 wild type and on a mutant strain in which
PSHAa0030
was inactivated. In
Ph
TAC125 wild-type cells,
PHSAa0030
is expressed at 4
C and 15
C.
PSHAa0458
and
PSHAa2217
encoding
the other TrHbs are expressed in both strains under all conditions, whereas
transcription of the FHb-encoding gene is detectable only in mutant cells
grown at 4
C in micro-aerobiosis (
Parrilli, Giuliani, Giordano, et al.,
2010
) (see
Section 6.2.4
).
To date, only group II of the TrHbs encoded by
PSHAa0030
(hereafter
named
Ph
-2/2HbO) has been thoroughly investigated from the structural
and functional viewpoints (
Coppola et al., 2013; Giordano et al., 2007,
2011; Howes et al., 2011; Parrilli, Giuliani, Giordano, et al., 2010; Russo
et al., 2013
). The gene was selected as the first of the three because its posi-
tion on chromosome I is very close to the origin of replication of the bac-
terium, indicating an important physiological role (
Giordano et al., 2007
).
Since transcription of FHb-encoding genes is usually directly or indi-
rectly induced by NO (
Hausladen et al., 1998; Spiro, 2007
), the observed
FHb-gene expression only in a
Ph
TAC125 mutant strain is suggestive of
occurrence of an NO-induced stress related to the absence of the TrHb
encoded by
PSHAa0030
(
Parrilli, Giuliani, Giordano, et al., 2010
; see
Section 6.2.4
).
6.2. Structure
-
function relationships of Ph-2/2HbO
6.2.1 Structure
Group II of TrHbs is by far the most populated of the three and is
characterised by specific residues building up the haem cavity (
Vuletich &
