Clostridium acetobutylicum and Clostridium aminovalericum, both obligatory anaerobes, grow normally after growth conditions are changed from anoxic to microoxic, where the cells consume
oxygen proficiently. In C. aminovalericum, a gene encoding a previously characterized H2O-forming
NADH oxidase, designated noxA, was cloned and sequenced. The expression of noxA was strongly upregulated within 10 min after the growth conditions were altered to a microoxic state, indicating that C. aminovalericum NoxA is involved in
oxygen metabolism. In C. acetobutylicum, genes suggested to be involved in
oxygen metabolism and genes for
reactive oxygen species (ROS) scavenging were chosen from the genome database. Although no clear orthologue of C. aminovalericum NoxA was found, Northern blot analysis identified many O2-responsive genes (e.g., a gene cluster [CAC2448 to CAC2452] encoding an
NADH rubredoxin oxidoreductase-A-type
flavoprotein-
desulfoferrodoxin homologue-MerR family-like
protein-
flavodoxin, an operon [CAC1547 to CAC1549] encoding a
thioredoxin-
thioredoxin reductase-
glutathione peroxidase-like
protein, an operon [CAC1570 and CAC1571] encoding two
glutathione peroxidase-like
proteins, and genes encoding
thiol peroxidase,
bacterioferritin comigratory
proteins, and
superoxide dismutase) whose expression was quickly and synchronously upregulated within 10 min after
flushing with 5% O2. The corresponding
enzyme activities, such as
NAD(P)H-dependent
peroxide (H2O2 and alkyl hydroperoxides)
reductase, were highly induced, indicating that microoxic growth of C. acetobutylicum is associated with the expression of a number of genes for
oxygen metabolism and ROS scavenging.