Changes in proteome of the Δhfq strain derived from Francisella tularensis LVS correspond with its attenuated phenotype

The posttranscriptional regulatory protein Hfq was shown to be an important determinant of the stress resistance and full virulence in the dangerous human pathogen Francisella tularensis. Transcriptomics brought rather limited clues to the precise contribution of Hfq in virulence. To reveal the molecular basis of the attenuation caused by hfq inactivation, we employed iTRAQ in the present study and compared proteomes of the parent and isogenic Delta hfq strains. We show that Hfq modulates the level of 76 proteins. Most of them show decreased abundance in the Delta hfq mutant, thereby indicating that Hfq widely acts rather as a positive regulator of Francisella gene expression. Several key Francisella virulence factors including those encoded within the Francisella pathogenicity island were found among the downregulated proteins, which is in a good agreement with the attenuated phenotype of the Delta hfq strain. To further validate the iTRAQ exploratory findings, we subsequently performed targeted LC-SRM analysis of selected proteins. This accurate quantification method corroborated the trends found in the iTRAQ data.