# Control of _Francisella tulareis_ Virulence at Gene Level: Network of Transcription Factors
## 4 Global Regulatory Proteins Important for _F. tulareis_ Virulence  
### The HU Protein: A Small Unit Necessary for F. tulareis Virulence  
Similar to eukaryotic histones, which are responsible for the alteration of the DNA structure and gene regulation, bacteria contain small basic regulatory proteins called histone-like proteins, more precisely nucleoid-associated proteins due to their localization in bacterial cells. It is presumed that they play the same role in the cell as their eukaryotic counterparts [93]. They bind DNA and by bending DNA chains they are able to alter gene expression. Among this group of DNA-binding proteins belongs HU (histone-like factor U), FIS (factor for inversion stimulation), H-NS (histone-like nucleoid structuring), and IHF (integration host factor) proteins [94]. HU and IHF proteins are homologous and are strongly conserved among bacterial species. Additionally, the HU protein enhances the DNA-binding capacity of IHF [95]. Their roles in the cell are indispensable and disruption or malfunction of one of them could lead to the alteration of basic metabolic pathways or even attenuation of virulence of pathogenic bacteria. Although the HU protein has been well studied in Gram-negative bacteria other than _F. tulareis_ [reviewed in Reference 47], its significance for _F. tulareis_ subsp. _holarctica_ intracellular growth and virulence was elucidated only recently [42]. HU forms heterodimers in most bacteria studied (e.g., Enterobacteriaceae) and is encoded by two genes _hupA_ and _hupB_[93], but in some bacteria (e.g., _F. tulareis_, _M. tuberculis_) [42; 96; 97], HU is encoded by a single _hupB_ gene and forms homodimers. The HU protein is known to be abundant in the bacterial cell [85], where it is able to influencerecombination, replication, transcription, and DNA shape [98; 99]. Moreover, the HU protein is capable to modulate the interaction between the IHF nucleoid-associated protein and its target DNA sequence in the _oriC_ gene; thereby, HU contributes to DNA replication in _E. coli_[100]. The HU protein binds all nucleic acids in a non-specific manner [101], but it shows higher preferences for abnormal structures, such as four-way junctions or replication forks [102]. By binding to DNA, HU ensures its negative supercoiling, whereby it contributes to the regulation of gene expression [103]. The HU-DNA interaction also provides protection of DNA against free hydroxyl radicals that could irreversibly damage DNA [42; 104]. DNA-binding sites of HU are of different lengths in various bacteria [95; 99; 105; 106; 107], but preferences for A/T rich sites have been identified [108]. It has been suggested that HU is not a specific transcription regulator, but rather that it acts as a separator of transcription units by its ability of DNA supercoiling, and thus it can influence gene transcription, either as a positive or negative regulator [109]. HU functions as a transcription factor inside the bacterial cell, but it is also secreted extracellularly. In Pseudomonas, HU was found as a component of biofilm formed by the bacterium [110]. It is also secreted by Wolbachia directly into the host cell nucleus, where it binds host DNA [111]. In _F. tulareis_ subsp. _holarctica_ LVS, HU was found to be secreted since it was identified in the culture filtrate [112] and constituted a minor component of secreted outer membrane vesicles (OMV) [18]. HU is necessary for the _F. tulareis_ subsp. _holarctica_ FSC200 strain intracellular replication and virulence in mice [42]. It was also proven that it is able to bind dsDNA and, thereby, protect against hydroxyl radicals. Deletion of the _lupB_ gene (encoding HU protein) led to the downregulation of PigR and several FPI proteins as well as to the downregulation of _pigR_ and several FPI genes. An _F. tulareis_ mutant strain lacking _lupB_ gene is attenuated in vitro and in vivo as well, demonstrating that HU is important for Francisella virulence [42]. Recently, [PERSON] et al.