Abstract

Plasmid addiction or Toxin-Antitoxin systems (TA systems)are responsible for post-segregational killing in bacteria when the plasmid harboring the system is lost.These systems are widespread and also a multitude of chromosomally located copies have been found.These might function as regulatory switches under stress conditions. We have mainly studied the structures and biophysics of the CcdA-CcdB (antitoxin /toxin) system. Here the toxin binds Gyrase and induces it to become a poison.The CcdA antitoxin inhibits CcdB’s action,it rejuvenates the Gyrase-Ccdb complex and binding of CcdB turns CcdA into a stronger autoregulatory transcription factor. The interactions systematically involve IDP regions that structure upon binding.The relevant peptides – corresponding to the intrinsically disordered part of CcdA – show remarkably high affinities,also rejuvenate the poisoned Gyrase by an allosteric zipper mechanism and this both in vitro and in vivo.