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Characterization of the Francisella tularensis subsp. novicida type IV pilus

Jirong Liu^1,

¹ South Texas Center for Emerging Infectious Diseases and Department of Biology, University of Texas San Antonio, San Antonio, TX 78249, USA
² Center for Infectious Diseases, Department of Molecular Genetics and Microbiology, Stony Brook University, Stony Brook, NY 11794, USA

Correspondence
Karl E. Klose
Karl.Klose{at}utsa.edu

Francisella tularensis causes the disease tularaemia. Type IV pili (Tfp) genes are present in the genomes of all F. tularensis subspecies. We show that the wild-type F. tularensis subsp. novicida expresses pilus fibres on its surface, and mutations in the Tfp genes pilF and pilT disrupt pilus biogenesis. Mutations in other Tfp genes (pilQ and pilG) do not eliminate pilus expression. A mutation in pilE4 eliminates pilus expression, whereas mutations in the other pilin subunits pilE1–3 and pilE5 do not, suggesting that pilE4 is the major pilus structural subunit. The virulence regulator MglA is required for pilus expression, and it regulates the transcription of a putative Tfp glycosylation gene (FTN0431). However, MglA does not regulate transcription of pilF, pilT or pilE4, and a strain lacking FTN0431 still expresses pili; thus, it is unclear how MglA regulates pilus expression. Only pilF was also required for protein secretion, while pilE4 and pilT were not, indicating that there is very little overlap of the protein secretion/Tfp functions of the pil genes. The protein secretion component pilE1 was more important for in vitro intramacrophage growth and mouse virulence than the Tfp component pilE4. Our results provide the first genetic characterization of the novel Tfp system of F. tularensis.

Present address: Beijing Great-Genius Science and Technology Company, Beijing 100089, PR China.

Two supplementary figures, showing the alignment of pilE4 genes from different F. tularensis subspecies and the virulence of F. tularensis subsp. novicida pilE1 and pilE4 strains via intranasal inoculation in mice (repeat experiment), and a supplementary table listing the PCR primers used, are available with the online version of this paper.

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