Biology Reference
In-Depth Information
6.2.4 NO detoxification
The remarkably high number of TrHbs in the
Ph
TAC125 genome strongly
suggests that these globins fulfil important functional roles associated with
the extreme features of the Antarctic habitat. The involvement of cold-
adapted
Ph
-2/2HbO in detoxification of RNS and ROS may be a mech-
anism associated with high production of toxic species upon cold stress.
A similar function has been reported in HbN of
M. tuberculosis
(
Pathania
et al., 2002
) and
M. bovis
(
Ouellett et al., 2002
) and
M. leprae
HbO
(
Fabozzi, Ascenzi, Renzi, &Visca, 2006
), which protect pathogenic bacteria
from the toxic activity of macrophage-generated RNS and ROS.
The physiological role of
Ph
-2/2HbO has been investigated using a geno-
mic approach, by the construction of a
Ph
TAC125 mutant strain in which the
PSHAa0030
gene was inactivated by insertional mutagenesis. The mutant
strain was grown under controlled conditions and its growth behaviour was
compared to that of wild-type cells, changing O
2
pressure in solution and
growth temperature (4 and 15
C). Regardless of temperature, growth of
the mutant strain in extreme aerobiosis is lower than that of the wild type, also
in terms of biomass. The presence of
Ph
-2/2HbO in wild-type cells is thus an
advantagewhen cells are grown at highO
2
concentration. Inmicro-aerobiosis,
both strains slow down their replication kinetics. At 4
C, the wild-type cells
appear better suited to the challenging conditions, reaching higher biomass
than the mutant cells (
Parrilli, Giuliani, Giordano, et al., 2010
).
The inactivation of the
Ph
-2/2HbO gene makes the mutant strain sen-
sitive to high O
2
levels, hydrogen peroxide and nitrosating agents (
Parrilli,
Giuliani, Giordano, et al., 2010
), suggesting involvement of the protein in
protection from oxidative and nitrosative stress. Moreover, the transcription
of the FHb-encoding gene occurs only in the mutant in which
PSHAa0030
is inactivated, when grown in micro-aerobiosis at 4
C, suggesting that the
occurrence of the NO-induced stress is probably related to the absence of
Ph
-2/2HbO (
Parrilli, Giuliani, Giordano, et al., 2010
). In micro-aerobiosis,
Ph
TAC125 may endogenously produce NO, due to a gene encoding a
nitrite reductase (
PSHAa1477
), as reported in other Gram-negative bacteria
(
Corker & Poole, 2003; Ji & Hollocher, 1988
). Further, NO may accumu-
late when its spontaneous oxidation is limited by low O
2
availability. In
micro-aerobiosis, O
2
is further reduced when the biomass is increased, that
is, in the late exponential phase, and NO accumulation may become a seri-
ous threat for cell viability. Induction of the FHb gene may be viewed as a
suitable strategy aimed at counteracting NO-induced stress due to the
absence of
Ph
-2/2HbO (
Parrilli, Giuliani, Giordano, et al., 2010
).