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Journal of Bacteriology, September 2008, p. 6234-6242, Vol. 190, No. 18
0021-9193/08/$08.00+0     doi:10.1128/JB.00794-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Characterization of Sec-Translocon-Dependent Extracytoplasmic Proteins of Rickettsia typhi{triangledown} ,{dagger}

Nicole C. Ammerman,* M. Sayeedur Rahman, and Abdu F. Azad

Department of Microbiology and Immunology, University of Maryland School of Medicine, 660 West Redwood Street, Howard Hall Suite 324, Baltimore, Maryland 21201

Received 6 June 2008/ Accepted 11 July 2008

As obligate intracellular, vector-borne bacteria, rickettsiae must adapt to both mammalian and arthropod host cell environments. Deciphering the molecular mechanisms of the interactions between rickettsiae and their host cells has largely been hindered by the genetic intractability of these organisms; however, research in other gram-negative pathogens has demonstrated that many bacterial determinants of attachment, entry, and pathogenesis are extracytoplasmic proteins. The annotations of several rickettsial genomes indicate the presence of homologs of the Sec translocon, the major route for bacterial protein secretion from the cytoplasm. For Rickettsia typhi, the etiologic agent of murine typhus, homologs of the Sec-translocon-associated proteins LepB, SecA, and LspA have been functionally characterized; therefore, the R. typhi Sec apparatus represents a mechanism for the secretion of rickettsial proteins, including virulence factors, into the extracytoplasmic environment. Our objective was to characterize such Sec-dependent R. typhi proteins in the context of a mammalian host cell infection. By using the web-based programs LipoP, SignalP, and Phobius, a total of 191 R. typhi proteins were predicted to contain signal peptides targeting them to the Sec translocon. Of these putative signal peptides, 102 were tested in an Escherichia coli-based alkaline phosphatase (PhoA) gene fusion system. Eighty-four of these candidates exhibited signal peptide activity in E. coli, and transcriptional analysis indicated that at least 54 of the R. typhi extracytoplasmic proteins undergo active gene expression during infections of HeLa cells. This work highlights a number of interesting proteins possibly involved in rickettsial growth and virulence in mammalian cells.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, University of Maryland School of Medicine, 660 West Redwood Street, Howard Hall Suite 324, Baltimore, MD 21201. Phone: (410) 706-3337. Fax: (410) 706-0282. E-mail: namme001{at}umaryland.edu

{triangledown} Published ahead of print on 18 July 2008.

{dagger} Supplemental material for this article may be found at http://jb.asm.org/.

Journal of Bacteriology, September 2008, p. 6234-6242, Vol. 190, No. 18
0021-9193/08/$08.00+0     doi:10.1128/JB.00794-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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