HU protein is involved in intracellular growth and full virulence of Francisella tularensis

Abstract

The nucleoid-associated HU proteins are small abundant DNA-binding proteins in bacterial cell which play an important role in the initiation of DNA replication, cell division, SOS response, control of gene expression and recombination. HU proteins bind to double stranded DNA non-specifically, but they exhibit high affinity to abnormal DNA structures as four-way junctions, gaps or nicks, which are generated during DNA damage. In many pathogens HU proteins regulate expression of genes involved in metabolism and virulence. Here, we show that the Francisella tularensis subsp. holarctica gene locus FTS_0886 codes for functional HU protein which is essential for full Francisella virulence and its resistance to oxidative stress. Further, our results demonstrate that the recombinant FtHU protein binds to double stranded DNA and protects it against free hydroxyl radicals generated via Fenton's reaction. Eventually, using an iTRAQ approach we identified proteins levels of which are affected by the deletion of hupB, among them for example Francisella pathogenicity island (FPI) proteins. The pleiotropic role of HU protein classifies it as a potential target for the development of therapeutics against tularemia.

Nucleoid Associated Proteins: The Small Organizers That Help to Cope With Stress

“…In S. coelicolor, sIHF (IHF homolog) and HupS (HU-like protein) are required to enable the DNA to fit inside the tiny spores (spores deprived of sIHF or HupS are temperature sensitive) (Salerno et al, 2009;Swiercz et al, 2013). An HU-like protein found in the human pathogen, Francisella tularensis (the causative agent of tularemia), protects the DNA against free hydroxyl radicals (Stojkova et al, 2018). A similar mechanism of action is exhibited by the Staphylococcus aureus MgrA protein; this homolog of E. coli Dps coats the DNA, protecting it against oxidative stress and ensuring prolonged survival of the cell inside phagosomes (Crosby et al, 2016;Ushijima et al, 2017).…”

“…Homologs of the E. coli H-NS and Fis proteins produced in the plant pathogen, Dickeya dadantii, influence the expression levels of the pal genes, which act as major virulence factors (Ouafa et al, 2012). Intriguingly, HU-like proteins found in Salmonella and F. tularensis not only create the physical protective barrier against stress factors, they also regulate genes involved in general physiology, metabolism, and virulence ( Figure 2B; Mangan et al, 2011;Stojkova et al, 2018). The mycobacterial HupB protein regulates the expression of the katG gene (acting as a repressor), whose product activates the anti-tuberculosis drug, isoniazid (Niki et al, 2012;Enany et al, 2017); a M. smegmatis strain deprived of HupB showed increased susceptibility to this drug (Hołówka et al, 2017).…”

Control of Francisella tularensis Virulence at Gene Level: Network of Transcription Factors

“…Although the HU protein has been well studied in Gram-negative bacteria other than F. tularensis [reviewed in Reference 47], its significance for F. tularensis 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. tularensis, M. tuberculosis ) [ 42 , 96 , 97 ], HU is encoded by a single hupB gene and forms homodimers.…”

“…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. tularensis, M. tuberculosis ) [ 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 influence recombination, replication, transcription, and DNA shape [ 98 , 99 ].…”

“…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 ].…”