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Journal of Bacteriology, October 1999, p. 5940-5947, Vol. 181, No. 19
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Functions of Two Types of NADH Oxidases in Energy Metabolism and Oxidative Stress of Streptococcus mutans

Masako Higuchi,1,* Yuji Yamamoto,1 Leslie B. Poole,2 Mamoru Shimada,3 Yutaka Sato,4 Nobuhiro Takahashi,5 and Yoshiyuki Kamio1

Department of Molecular and Cell Biology, Division of Life Science, Graduate School of Agriculture, Tohoku University, Aoba-ku, Sendai 981-8555,1 Research Center, Nippon Paint Co., Ltd., Neyagawa, Osaka 572-0074,3 Department of Biochemistry, Tokyo Dental College, Mihama-ku, Chiba 261-0022,4 and Department of Oral Biochemistry, Tohoku University School of Dentistry, Aoba-ku, Sendai 980-8575,5 Japan, and Department of Biochemistry, Wake Forest University Medical School, Winston-Salem, North Carolina 271572

Received 8 April 1999/Accepted 22 July 1999

We have previously identified two distinct NADH oxidases corresponding to H2O2-forming oxidase (Nox-1) and H2O-forming oxidase (Nox-2) induced in Streptococcus mutans. Sequence analyses indicated a strong similarity between Nox-1 and AhpF, the flavoprotein component of Salmonella typhimurium alkyl hydroperoxide reductase; an open reading frame upstream of nox-1 also showed homology to AhpC, the direct peroxide-reducing component of S. typhimurium alkyl hydroperoxide reductase. To determine their physiological functions in S. mutans, we constructed knockout mutants of Nox-1, Nox-2, and/or the AhpC homologue; we verified that Nox-2 plays an important role in energy metabolism through the regeneration of NAD+ but Nox-1 contributes negligibly. The Nox-2 mutant exhibited greatly reduced aerobic growth on mannitol, whereas there was no significant effect of aerobiosis on the growth on mannitol of the other strains or growth on glucose of any of the strains. Although the Nox-2 mutants grew well on glucose aerobically, the end products of glucose fermentation by the Nox-2 mutant were substantially shifted to higher ratios of lactic acid to acetic acid compared with wild-type cells. The resistance to cumene hydroperoxide of Escherichia coli TA4315 (ahpCF-defective mutant) transformed with pAN119 containing both nox-1 and ahpC genes was not only restored but enhanced relative to that of E. coli K-12 (parent strain), indicating a clear function for Nox-1 as part of an alkyl hydroperoxide reductase system in vivo in combination with AhpC. Surprisingly, the Nox-1 and/or AhpC deficiency had no effect on the sensitivity of S. mutans to cumene hydroperoxide and H2O2, implying that the existence of some other antioxidant system(s) independent of Nox-1 in S. mutans compensates for the deficiency.

* Corresponding author. Mailing address: Department of Molecular and Cell Biology, Graduate School of Agriculture, Tohoku University, Aoba-ku, Sendai 981-8555, Japan. Phone: 81-22-717-8781. Fax: 81-22-717-8780. E-mail: mhiguchi{at}biochem.tohoku.ac.jp.

Journal of Bacteriology, October 1999, p. 5940-5947, Vol. 181, No. 19
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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