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

The toxin-antitoxin operon of pSM19035 encodes three proteins: the omega global regulator, the E labile antitoxin and the stable zeta toxin. Accumulation of toxin free of e antitoxin induced loss of cell proliferation in both Bacillus subtilis and Escherichia coli cells. Induction of a variant (zeta Y83C) 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 zeta toxin induced stasis, but this was fully reversed by expression of the epsilon antitoxin within a defined time window. The time window for reversion of zeta toxicity by expression of epsilon antitoxin was dependent on the initial cellular level of zeta. After 240 min of constitutive expression, or inducible expression of high levels of C toxin for 30 min, expression of E failed to reverse the toxic effect exerted by zeta in cells growing in minimal medium. Under the latter conditions, zeta inhibited replication, transcription and translation and finally induced death in a fraction (similar to 50 %) of the cell population. These results support the view that zeta interacts with its specific target and reversibly inhibits cell proliferation, but accumulation of C might lead to cell death due to pleiotropic effects.