Crystal structure of the plasmid maintenance system ε /ζ : Functional mechanism of toxin ζ and inactivation by ε 2ζ 2 complex formation

Programmed cell death in prokaryotes is frequently found as postsegregational killing. It relies on antitoxin/toxin systems that secure stable inheritance of low and medium copy number plasmids during cell division and kill cells that have lost the plasmid. The broad−host−range, low−copy−number plasmid pSM19035 from Streptococcus pyogenes carries the genes encoding the antitoxin/toxin system varε /ζ and antibiotic resistance proteins, among others. The crystal structure of the biologically nontoxic varε 2ζ 2 protein complex at a 1.95−Å resolution and site−directed mutagenesis showed that free ζacts as phosphotransferase by using ATP/GTP. In varepsilon 2ζ 2, the toxin zeta is inactivated because the N−terminal helix of the antitoxin varepsilon blocks the ATP/GTP−binding site. To our knowledge, this is the first prokaryotic postsegregational killing system that has been entirely structurally characterize