Identification of a Novel Small RNA Modulating <em>Francisella tularensis</em> Pathogenicity

<div><p><em>Francisella tularensis</em> is a highly virulent bacterium responsible for the zoonotic disease tularemia. It is a facultative intracellular pathogen that replicates in the cytoplasm of host cells, particularly in macrophages. Here we show that <em>F. tularensis</em> live vaccine strain (LVS) expresses a novel small RNA (sRNA), which modulates the virulence capacities of the bacterium. When this sRNA, designated FtrC (for <em><u>F</u>rancisella <u>t</u>ularensis</em><u>R</u>NA <u>C</u>), is expressed at high levels, <em>F. tularensis</em> replicates in macrophages less efficiently than the wild-type parent strain. Similarly, high expression of FtrC reduces the number of viable bacteria recovered from the spleen and liver of infected mice. Our data demonstrate that expression of gene <em>FTL_1293</em> is regulated by FtrC. Furthermore, we show by <em>in vitro</em> gel shift assays that FtrC interacts specifically with <em>FTL_1293</em> mRNA and that this happens independently of the RNA chaperone Hfq. Remarkably, FtrC interacts only with full-length <em>FTL_1293</em> mRNA. These results, combined with a bioinformatic analysis, indicate that FtrC interacts with the central region of the mRNA and hence does not act by sterically hindering access of the ribosome to the mRNA. We further show that gene <em>FTL_1293</em> is not required for <em>F. tularensis</em> virulence <em>in vitro</em> or <em>in vivo</em>, which indicates that another unidentified FtrC target modulates the virulence capacity of the bacterium.</p>