The molecular basis of HicA dependent binding of HicB to DNA

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Toxin-antitoxin (TA) systems are typically encoded as bicistronic operons in a variety of bacterial species and archaea and act as response effectors to a variety of intra and extra-cellular stressors. TA members are classed into 6 generic systems and type 2 TA systems are able to directly autoregulate their own expression via a series of complex interactions between the protein toxin, the protein antitoxin and the DNA upstream of the encoding genes. This ensures that in a non-stress environment, the protein antitoxin is in excess of the protein toxin resulting in neutralisation. During a state of stress, the intracellular toxin concentration accumulates and the toxin is able to exert its toxicity, typically resulting in bacteriostasis. This mechanism has been implicated in the generation of antibiotic tolerant (persister) cells, which can result in chronic infections. The HicAB system is one of the prototypic type 2 TA systems found in several bacteria and archaea, yet remains underinvestigated when compared to other TA systems. This thesis reports structural studies on the Burkholderia pseudomallei HicAB TA pair and associated DNA interactions. HicB forms a tetramer and this can interact with DNA upstream of the bicistronic hicAB operon to form a tight binding complex. HicA and HicB form a hetero-octameric complex that involves structural re-organisation of the carboxy-terminal (DNA binding) region of HicB. HicA therefore has a profound impact on binding of HicB to DNA sequences upstream of hicAB, in a stoichiometric-dependent way. At low ratios of HicA:HicB there is no effect on DNA binding, but at higher ratios the affinity for DNA declines co-operatively, driving dissociation of the HicA:HicB:DNA complex. This provides the first example of the molecular mechanisms by which HicA de-represses a HicB-DNA complex and is a distinct mechanism when compared to other TA family members investigated to date
Date of Award23 Jan 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMatthew P Crump (Supervisor)

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