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Journal of Bacteriology, June 2007, p. 4343-4352, Vol. 189, No. 12
0021-9193/07/$08.00+0     doi:10.1128/JB.00010-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Structural Properties of Periplasmic SodCI That Correlate with Virulence in Salmonella enterica Serovar Typhimurium

Radha Krishnakumar,^1, Byoungkwan Kim,¹ Elizabeth A. Mollo,¹ James A. Imlay,¹ and James M. Slauch^1,2*

Department of Microbiology,¹ College of Medicine, University of Illinois, Urbana, Illinois 61801²

Received 2 January 2007/ Accepted 22 March 2007

Salmonella enterica strains survive and propagate in macrophages by both circumventing and resisting the antibacterial effectors normally delivered to the phagosome. An important aspect of Salmonella resistance is the production of periplasmic superoxide dismutase to combat phagocytic superoxide. S. enterica serovar Typhimurium strain 14028 produces two periplasmic superoxide dismutases: SodCI and SodCII. Both enzymes are produced during infection, but only SodCI contributes to virulence in the animal. Although 60% identical to SodCII at the amino acid level with very similar enzymatic properties, SodCI is dimeric, protease resistant, and tethered within the periplasm via a noncovalent interaction. In contrast, SodCII is monomeric and protease sensitive and is released from the periplasm normally by osmotic shock. We have constructed an enzymatically active monomeric SodCI enzyme by site-directed mutagenesis. The resulting protein was released by osmotic shock and sensitive to protease and could not complement the loss of wild-type dimeric SodCI during infection. To distinguish which property is most critical during infection, we cloned and characterized related SodC proteins from a variety of bacteria. Brucella abortus SodC was monomeric and released by osmotic shock but was protease resistant and could complement SodCI in the animal. These data suggest that protease resistance is a critical property that allows SodCI to function in the harsh environment of the phagosome to combat phagocytic superoxide. We propose a model to account for the various properties of SodCI and how they contribute to bacterial survival in the phagosome.

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* Corresponding author. Mailing address: Department of Microbiology, University of Illinois, B103 Chemical and Life Sciences Laboratory, 601 S. Goodwin Ave., Urbana, IL 61801. Phone: (217) 244-1956. Fax: (217) 244-6697. E-mail: slauch{at}uiuc.edu

Published ahead of print on 6 April 2007.

Present address: The J. Craig Venter Institute, 9704 Medical Center Drive, Rockville, MD 20850.

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Journal of Bacteriology, June 2007, p. 4343-4352, Vol. 189, No. 12
0021-9193/07/$08.00+0     doi:10.1128/JB.00010-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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