Doc of prophage P1 is inhibited by its antitoxin partner Phd though fold complementation

Prokaryotic toxin-antitoxin (TA) modules are involved in major physiological events set in motion under stress conditions. The crystal structure of the toxin Doc (death on curing) from the Phd/Doc module on phage P1 shows a fold characteristic of the Fic (filamentation induced by cAMP) proteins that is made complete by the C-terminal domain of its antitoxin partner Phd (prevents host death). This domain, intrinsically disordered in solution, folds into an alpha-helix upon binding to Doc. The details of these interactions reveal the molecular basis for the inhibitory action of the antitoxin and suggest a possible origin for this TA operon. Doc induces growth arrest of E. coli cells in a reversible manner, by targeting the protein synthesis machinery. Doc activates the endogenous E. coli RelE mRNA interferase but does not require this or any other known chromosomal TA locus for its action in vivo.