Rejuvenation of CcdB-poisoned gyrase by an intrinsically disordered protein domain

Toxin-antitoxin modules are small regulatory circuits that ensure survival of bacterial populations under challenging environmental conditions. The ccd toxin-antitoxin module on the F plasmid codes for
the toxin CcdB and its antitoxin CcdA. CcdB poisons gyrase while CcdA actively dissociates CcdB:gyrase
complexes in a process called rejuvenation. The CcdA:CcdB ratio modulates autorepression of the
ccd operon. The mechanisms behind both rejuvenation and regulation of expression are poorly understood.
We show that CcdA binds consecutively to two partially overlapping sites on CcdB, which differ in affinity by six orders of magnitude. The first, picomolar affinity interaction triggers a conformational
change in CcdB that initiates the dissociation of CcdB:gyrase complexes by an allosteric segmental
binding mechanism. The second, micromolar affinity binding event regulates expression of the ccd
operon. Both functions of CcdA, rejuvenation and autoregulation, are mechanistically intertwined and
depend crucially on the intrinsically disordered nature of the CcdA C-terminal domain.