Balancing at survival's edge: the structure and adaptive benefits of prokaryotic toxin-antitoxin partners.

Many prokaryotes express toxin-antitoxin (TA) pairs that are harmful to their hosts if not maintained in delicate balance. The maintenance of potentially lethal toxin-antitoxin pairs could be viewed as a high-risk strategy. However, accumulating evidence suggests that toxin-antitoxin pairs can confer selective evolutionary benefits such as adaptive stress responses, starvation recovery and herd immunity to predation. Many of the known TA pairs interact as proteins, but recent work has identified a new class of antitoxins that are RNA cleavage products. Structural studies have revealed common folds for diverse toxins, highlighting unexpected evolutionary relationships within different toxin classes. TA pairs appear to have diverged in function considerably, to meet the specialised requirements of their varied prokaryotic hosts.