Fuzzy recognition by the prokaryotic transcription factor HigA2 from Vibrio cholerae

Disordered protein sequences can exhibit different binding modes, ranging from well-ordered folding-upon-binding to highly dynamic fuzzy binding. The primary function of the intrinsically disordered region (IDR) of the antitoxin HigA2 from Vibrio cholerae is to neutralize HigB2 toxin through ultra-high-affinity folding-upon-binding interaction. Here, we show that the same IDR can also mediate fuzzy interactions with its operator DNA and, through interplay with the folded helix-turn-helix domain, regulates transcription from the higBA2 operon. NMR, SAXS, ITC and in vivo experiments converge towards a consistent picture where a specific set of residues in the IDR mediate electrostatic and hydrophobic interactions while hovering over the DNA operator. Sensitivity of the IDR to scrambling the sequence, position-specific contacts and absence of redundant, multivalent interactions, point towards a novel, more specific type of fuzzy binding. Our work demonstrates how a bacterial regulator achieves dual functionality by utilizing two distinct interaction modes within the same disordered sequence.