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Infection and Immunity, October 2004, p. 5983-5992, Vol. 72, No. 10
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.10.5983-5992.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Legionella pneumophila DotU and IcmF Are Required for Stability of the Dot/Icm Complex

Jessica A. Sexton,1 Jennifer L. Miller,1 Aki Yoneda,1 Thomas E. Kehl-Fie,2 and Joseph P. Vogel1*

Department of Molecular Microbiology,1 Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri2

Received 7 April 2004/ Returned for modification 13 May 2004/ Accepted 26 June 2004

Legionella pneumophila utilizes a type IV secretion system (T4SS) encoded by 26 dot/icm genes to replicate inside host cells and cause disease. In contrast to all other L. pneumophila dot/icm genes, dotU and icmF have homologs in a wide variety of gram-negative bacteria, none of which possess a T4SS. Instead, dotU and icmF orthologs are linked to a locus encoding a conserved cluster of proteins designated IcmF-associated homologous proteins, which has been proposed to constitute a novel cell surface structure. We show here that dotU is partially required for L. pneumophila intracellular growth, similar to the known requirement for icmF. In addition, we show that dotU and icmF are necessary for optimal plasmid transfer and sodium sensitivity, two additional phenotypes associated with a functional Dot/Icm complex. We found that these effects are due to the destabilization of the T4SS at the transition into the stationary phase, the point at which L. pneumophila becomes virulent. Specifically, three Dot proteins (DotH, DotG, and DotF) exhibit decreased stability in a {Delta}dotU {Delta}icmF strain. Furthermore, overexpression of just one of these proteins, DotH, is sufficient to suppress the intracellular growth defect of the {Delta}dotU {Delta}icmF mutant. This suggests a model where the DotU and IcmF proteins serve to prevent DotH degradation and therefore function to stabilize the L. pneumophila T4SS. Due to their wide distribution among bacterial species and their genetic linkage to known or predicted cell surface structures, we propose that this function in complex stabilization may be broadly conserved.

* Corresponding author. Mailing address: Department of Molecular Microbiology, Washington University, Campus Box 8230, 660 S. Euclid Ave., St. Louis, MO 63110. Phone: (314) 747-1029. Fax: (314) 362-3203. E-mail: jvogel{at}borcim.wustl.edu.

Editor: J. T. Barbieri

Infection and Immunity, October 2004, p. 5983-5992, Vol. 72, No. 10
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.10.5983-5992.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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