Arginine 58 is indispensable for proper function of the <i>Francisella tularensis</i> subsp. <i>holarctica</i> FSC200 HU protein, and its substitution alters virulence and mediates immunity against wild-type strain.

HU protein, a member of the nucleoid-associated group of proteins, is an important transcription factor in bacteria, including in the dangerous human pathogen <i>Francisella tularensis</i>. Generally, HU protein acts as a DNA sequence non-specific binding protein and it is responsible for winding of the DNA chain that leads to the separation of transcription units. Here, we identified potential HU protein binding sites using the ChIP-seq method and two possible binding motifs in <i>F. tularensis</i> subsp. <i>holarctica</i> FSC200 depending upon growth conditions. We also confirmed that FSC200 HU protein is able to introduce negative supercoiling of DNA in the presence of topoisomerase I. Next, we showed interaction of the HU protein with a DNA region upstream of the <i>pigR</i> gene and inside the <i>clpB</i> gene, suggesting possible regulation of PigR and ClpB expression. Moreover, we showed that arginine 58 and partially arginine 61 are important for HU protein's DNA binding capacity, negative supercoiling induction by HU, and the length and winding of FSC200 chromosomal DNA. Finally, in order to verify biological function of the HU protein, we demonstrated that mutations in arginine 58, arginine 61, and serine 74 of the HU protein decrease virulence of FSC200 both <i>in vitro</i> and <i>in vivo</i> and that immunization using these mutant strains is able to protect as many as 100% of mice against wild-type challenge. Taken together, our findings deepen knowledge about the role of the HU protein in tularaemia pathogenesis and suggest that HU protein should be addressed in the context of tularaemia vaccine development.