# Control of _Francisella tulareis_ Virulence at Gene Level: Network of Transcription Factors
## 2 Transcription Factors Involved in Regulation of the FPI
### PigR (FevR) and Guanosine Testraphosphate (ppGpp) Interact with the MgIA/SspA Complex  
Following the discovery of the MgIA/SspA regulatory complex, the detailed molecular mechanisms on how this complex interacts with DNA were elucidated. It was shown that FevR (Francisella effector of virulence regulation) is required for expression of FPI genes from the MgIA/SspA regulon, and that the expression of _fevR_ is upregulated by MgIA and FevR/MgIA together upregulated virulence gene expression. Subsequently, the importance of FevR for intracellular replication of _F. novicida_ and its virulence in vivo was described [31]. A later study confirmed the involvement of FevR in regulating FPI gene expression also in _F. tulareis_, where the ortholog was denoted as PigR. It was confirmed that there was direct interaction between PigR and the MgIA/SspA complex and that this interaction was promoted by guanosine tetraphosphate (ppGpp) [32] in the presence of Mg\({}^{2+}\)[39]. A later study showed that MgIA/SspA must be bound to ppGpp to mediate high-affinity interactions with PigR, and that the MgIA/SspA complex interacts with the last 22 residues of the PigR-terminal domain [39]. The importance of the interaction between PigR and the MgIA/SspA complex has been demonstrated \begin{table}
\begin{tabular}{l l l l} \hline \hline \multicolumn{1}{c}{**Locus Tag**} & **Gene** & **Description** & **Reference** \\ \hline FIT\_1344 & _pdpA_ & FPI, Pathogenicity determinant protein A & [28] \\ FIT\_1345 (FTN\_1310) & _pdpA_ & FPI, Pathogenicity determinant protein B & [28; 29] \\ FIT\_1346 & _igIE_ & FPI, Intracellular growth locus E & [28] \\ FIT\_1347 & _vgrG_ & FPI, Valine-glycine repeat protein G & [28] \\ FIT\_1348 & _igIF_ & FPI, Intracellular growth locus F & [28] \\ FIT\_1349 & _igIG_ & FPI, Intracellular growth locus G & [28] \\ FIT\_1350 & _igIH_ & FPI, Intracellular growth locus H & [28] \\ FIT\_1351 & _dotU_ & FPI, DotU domain-containing protein & [28] \\ FIT\_1352 & _igI_ & FPI, Intracellular growth locus I & [28] \\ FIT\_1353 & _igI_ & FPI, Intracellular growth locus J & [28] \\ FIT\_1354 & _pdpC_ & FPI, Pathogenicity determinant protein C & [28] \\ FIT\_1355 & _pdpE_ & FPI, Pathogenicity determinant protein E & [28] \\ FIT\_1356c & _igI_D & FPI, Intracellular growth locus D & [28] \\ FIT\_1357c (FTN\_1322) & _igIC_ & FPI, Intracellular growth locus C & [28; 29] \\ FIT\_1358c (FTN\_1323) & _igIB_ & FPI, Intracellular growth locus B & [28; 29] \\ FIT\_1359c (FTN\_1324) & _igIA_ & FPI, Intracellular growth locus A & [28; 29] \\ FIT\_1360c (FTN\_1325) & _pdpD_ & FPI, Pathogenicity determinant protein D & [28; 29] \\ FIT\_1051 & _hfq_ & Host factor for bacteriophage QB RNA replication & [29] \\ FIT\_0549 & _sspA_ & Stringent starvation protein A & [29] \\ FIT\_1054 & _hupB_ & DNA-binding protein HU-beta & [29] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Genes that are associated with _Francisella tulareis_ or _Francisella novicida_ virulence and are positively regulated by macrophage growth locus, subunit A (MgIA). to be necessary for the coordinate control of virulence genes [31; 32; 37]. Another signal molecule, inorganic polyphosphate (polyP), was identified to be important for MgIa/SspA and interactions between target genes and promoters. It was shown that the absence of polyP led to decreased binding of the MgIa/SspA complex to the promoters of the pathogenicity determinant protein D (_pdpD_), intracellular growth locus A (_igIa_), _fevR_, and _ppK_ genes [40]. Although PigR is able to associate with many promoters, only promoters that contain a specific sequence motif referred to as the PigR-response element (PRE) are positively regulated by PigR/MgIa/SspA [38]. In _E. coli_, ppGpp is a small regulating molecule that binds directly to the RNA polymerase (RNAP) and could thus both positively and negatively regulate transcription of genes [73]. The level of ppGpp in the bacterial cell is regulated by the ppGpp synthetase (RelA) and a bifunctional protein that is able to synthesize and degrade ppGpp, named SpoT [74].