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Journal of Bacteriology, March 2005, p. 2020-2029, Vol. 187, No. 6
0021-9193/05/$08.00+0     doi:10.1128/JB.187.6.2020-2029.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Cytochrome bd Oxidase, Oxidative Stress, and Dioxygen Tolerance of the Strictly Anaerobic Bacterium Moorella thermoacetica

Amaresh Das,1 Radu Silaghi-Dumitrescu,2 Lars G. Ljungdahl,1 and Donald M. Kurtz Jr.2*

Department of Biochemistry and Molecular Biology,1 Department of Chemistry and Center for Metalloenzyme Studies, University of Georgia, Athens, Georgia2

Received 17 September 2004/ Accepted 17 November 2004

The gram-positive, thermophilic, acetogenic bacterium Moorella thermoacetica can reduce CO2 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 O2 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 (~70%) inhibited both the endogenous and CO/methanol-dependent O2 uptake. UV-visible light absorption and electron paramagnetic resonance spectra of n-dodecyl-ß-maltoside extracts of M. thermoacetica membranes showed the presence of a cytochrome bd oxidase complex containing cytochrome b561, cytochrome b595, 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 + H2S). 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.

* Corresponding author. Mailing address: Department of Chemistry, University of Georgia, Athens, GA 30602-2556. Phone: (706) 542-2016. E-mail: kurtz{at}chem.uga.edu.

Journal of Bacteriology, March 2005, p. 2020-2029, Vol. 187, No. 6
0021-9193/05/$08.00+0     doi:10.1128/JB.187.6.2020-2029.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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