Antitoxin control of optimal transcriptional repression in the atypical HigB–HigA toxin–antitoxin system from <i>Proteus vulgaris</i>

Bacterial toxin-antitoxin (TA) pairs transcriptionally autoregulate their expression via a repression/derepression mechanism in response to changing environmental conditions. The structural diversity of TA systems influences the mechanisms of transcriptional regulation. Here, we define the molecular mechanism for the plasmid-encoded HigB-HigA TA pair originally identified in a post-operative infection with antibiotic-resistant Proteus vulgaris. We determine DNA binding and promoter activity by the HigB-HigA complex supported by structural biology and molecular dynamics simulations of an elusive DNA operator-TA repressor complex. To define the optimal oligomeric TA repressor-DNA operator complex required for derepression, we engineered a dedicated trimeric HigB-HigA2 complex that represses transcription more than 26-fold as compared to the tetrameric HigB2-HigA2. These results expand the known diversity of how the HigB-HigA TA family is autoregulated.