Cleavage of the antitoxin of the parD toxin-antitoxin system is determined by the ClpAP protease and is modulated by the relative ratio of the toxin and the antitoxin.

Differential stability of toxins and antitoxins is the key for the conditional activation and function of Toxin-Antitoxin systems. Here we report the evaluation of the action of cell proteases Lon, ClpAP, ClpXP and ClpYQ on the Kis antitoxin and the Kid toxin of the parD TA system of plasmid R1. In vitro analysis shows that Kis antitoxin, but not the Kid toxin, is cleaved specifically by the ClpAP protease. The Kid toxin is not cleaved either by this protease or by any of the others cell proteases tested but in complex with the Kis antitoxin protects the cleavage of this protein in a way that is dependent on the toxin-antitoxin ratio. We further show that this protection is correlated with the inability of the ClpA chaperone to access the Kis antitoxin when in complex with Kid toxin. The stability of the antitoxin greatly increases in vivo in a clpP- background and plasmid maintenance mediated by the parD system, which is dependent on the differential decay of the antitoxin, is reduced to the levels observed in the absence of a functional toxin. The functional implications of these data are further discussed within the frame of the regulation of the parD system and of the available information on the nature of the toxin-antitoxin complexes formed at different toxin-antitoxin ratios.