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to oxidative stress, suggesting a role for the globin in the oxidative stress
response. This proposal was later rejected (
Buisson & Labbe-Bois, 1998
)
on the grounds that globin expression is unchanged on exposure to
antimycin A or menadione, but decreased by hydrogen peroxide and other
reagents. Indeed, using a
YHB1
deletion strain, it was shown that YHb1 not
only protects cells against Cu(II) and dithiothreitol but also sensitizes them to
hydrogen peroxide (
Buisson & Labbe-Bois, 1998
).
Subsequently, however, other groups clearly established up-regulation
of the YHb by NO, and a role in protection from nitrosative stress (
Liu,
Zeng, Hausladen, Heitman, & Stamler, 2000
).
Lewinska and Bartosz
(2006)
later published similar results. It now seems likely that the pre-
eminent role of the
S. cerevisiae
globin is a protection against nitrosative
stress. FHb is located in both the cytosol and the mitochondrial matrix of
normoxic cells but exclusively in the mitochondria in the absence of oxy-
gen. Localization of FHb in the mitochondrial matrix suggests a role for this
globin as an NO-detoxifying system under hypoxic conditions by control-
ling levels of NO that suppress respiration by inhibiting cytochrome
c
oxi-
dase (
Cassanova, O'Brien, Stahl, McClure, & Poyton, 2005
).
The protective function of FHbs from nitrosative stress also occurs in path-
ogenic yeasts, such as
Cryptococcus neoformans
(
de Jesus-Berrios et al., 2003
)and
Candida albicans
, the most prevalent human fungal pathogen (
Ullmann et al.,
2004
). Curiously, only one of the three FHb genes in
C
.
albicans
is responsible
for NO consumption and detoxification (
Ullmann et al., 2004
). The gene
most highly up-regulated in the presence of NO is
YHB1
, and strains with
a deletion in this gene show hypersensitivity toNO and are highly filamentous
(
Hromatka, Noble, & Johnson, 2005
). In the case of
C
.
neoformans
,itwas
shown recently that production of NADPH via the NADP(
รพ
)-dependent
isocitrate dehydrogenase played an important role in the maintenance of
resistance to nitrosative stress (
Brown et al., 2009
).
6.3. Other fungi
The true fungi are eukaryotes and all are heterotrophs, that is, they rely on
external sources of organic carbon compounds for biosynthesis and survival.
Fungi recycle the biomass of other organisms, including biomaterials such as
lignin, wood and leaves that other organisms may be unable to digest. Many
of the true fungi represented in
Tables 9.4
and Supplementary Table S2 at
tant plant pathogens and therefore exert a high impact economically.
