Unique Substrates Secreted by the Type VI Secretion System of <em>Francisella tularensis</em> during Intramacrophage Infection

<div><p>Gram-negative bacteria have evolved sophisticated secretion machineries specialized for the secretion of macromolecules important for their life cycles. The Type VI secretion system (T6SS) is the most widely spread bacterial secretion machinery and is encoded by large, variable gene clusters, often found to be essential for virulence. The latter is true for the atypical T6SS encoded by the <em>Francisella</em> pathogenicity island (FPI) of the highly pathogenic, intracellular bacterium <em>Francisella tularensis</em>. We here undertook a comprehensive analysis of the intramacrophage secretion of the 17 FPI proteins of the live vaccine strain, LVS, of <em>F. tularensis</em>. All were expressed as fusions to the TEM β-lactamase and cleavage of the fluorescent substrate CCF2-AM, a direct consequence of the delivery of the proteins into the macrophage cytosol, was followed over time. The FPI proteins IglE, IglC, VgrG, IglI, PdpE, PdpA, IglJ and IglF were all secreted, which was dependent on the core components DotU, VgrG, and IglC, as well as IglG. In contrast, the method was not directly applicable on <em>F. novicida</em> U112, since it showed very intense native β-lactamase secretion due to FTN_1072. Its role was proven by ectopic expression <em>in trans</em> in LVS. We did not observe secretion of any of the LVS substrates VgrG, IglJ, IglF or IglI, when tested in a FTN_1072 deficient strain of <em>F. novicida</em>, whereas IglE, IglC, PdpA and even more so PdpE were all secreted. This suggests that there may be fundamental differences in the T6S mechanism among the <em>Francisella</em> subspecies. The findings further corroborate the unusual nature of the T6SS of <em>F. tularensis</em> since almost all of the identified substrates are unique to the species.</p>