pSM19035-encoded zeta toxin induces stasis followed by death in a subpopulation of cells.

The toxin–antitoxin operon of pSM19035 encodes three proteins: the ω global regulator, the e labile antitoxin and the stable ζ toxin. Accumulation of ζ toxin free of e antitoxin induced loss of cell proliferation in both Bacillus subtilis and Escherichia coli cells. Induction of a ζ variant (ζY83C) triggered stasis, in which B. subtilis cells were viable but unable to proliferate, without selectively affecting protein translation. In E. coli cells, accumulation of free ζ toxin induced stasis, but this was fully reversed by expression of the e antitoxin within a defined time window. The time window for reversion of ζ toxicity by expression of e antitoxin was dependent on the initial cellular level of ζ. After 240 min of constitutive expression, or inducible expression of high levels of ζ toxin for 30 min, expression of e failed to reverse the toxic effect exerted by ζ in cells growing in minimal medium. Under the latter conditions, ζ inhibited replication, transcription and translation and finally induced death in a fraction (∼50 %) of the cell population. These results support the view that ζ interacts with its specific target and reversibly inhibits cell proliferation, but accumulation of ζ might lead to cell death due to pleiotropic effects.