The gram-positive, thermophilic, acetogenic bacterium Moorella thermoacetica can reduce CO 2 to acetate via the Wood-Ljungdahl (acetyl coenzyme A synthesis) pathway. This report demonstrates that, despite its classification as a strict anaerobe, M. thermoacetica contains a membrane-bound cytochrome bd oxidase that can catalyze reduction of low levels of dioxygen. Whole-cell suspensions of M. thermoacetica had significant endogenous O 2 uptake activity, and this activity was increased in the presence of methanol or CO, which are substrates in the Wood-Ljungdahl pathway. Cyanide and azide strongly (ϳ70%) inhibited both the endogenous and CO/methanol-dependent O 2 uptake. UV-visible light absorption and electron paramagnetic resonance spectra of n-dodecyl--maltoside extracts of M. thermoacetica membranes showed the presence of a cytochrome bd oxidase complex containing cytochrome b 561 , cytochrome b 595 , and cytochrome d (chlorin). Subunits I and II of the bd oxidase were identified by N-terminal amino acid sequencing. The M. thermoacetica cytochrome bd oxidase exhibited cyanide-sensitive quinol oxidase activity. The M. thermoacetica cytochrome bd (cyd) operon consists of four genes, encoding subunits I and II along with two ABC-type transporter proteins, homologs of which in other bacteria are required for assembly of the bd complex. The level of this cyd operon transcript was significantly increased when M. thermoacetica was grown in the absence of added reducing agent (cysteine ؉ H 2 S). Expression of a 35-kDa cytosolic protein, identified as a cysteine synthase (CysK), was also induced by the nonreducing growth conditions. The combined evidence indicates that cytochrome bd oxidase and cysteine synthase protect against oxidative stress and contribute to the limited dioxygen tolerance of M. thermoacetica.Dioxygen is the preferred electron sink in the respiratory chain of most aerobic bacteria (58). Cytochrome oxidases are the membrane-bound, terminal components of the dioxygendependent respiratory chain, which reduces dioxygen to water with formation of pH and potential gradients (6, 45). Two main types of respiratory cytochrome oxidases are known in bacteria: the heme/copper oxidases (cytochrome aa 3 , cytochrome caa 3 oxidase, and cytochrome bo oxidase), and the heme-only cytochrome bd quinol oxidase, which is associated with microaerobic dioxygen respiration (24,56,59,62). Cytochrome bd oxidases purified from aerobic bacteria have been characterized as 1:1 heterodimers of two integral membrane proteins referred to as subunits I and II (35,55). The heterodimers contain three heme components: a low-spin heme, b 558 , and two high-spin hemes, b 595 and d. In the genomes of Escherichia coli (31) and Azotobacter vinelandii (41), subunits I and II are encoded by cydA and cydB, respectively, of the cytochrome bd (cyd) operon. Two ABC-type transporter proteins, CydD and CydC, are typically required for assembly of the cytochrome bd complex in both gram-positive and gram-negative aerobic bacteria (14, 60). Aerobic r...