Type III toxin-antitoxin loci

The toxIN locus from a cryptic plasmid of the plant pathogen Pectobacterium atrosepticum was the first type III toxin-antitoxin (TA) system to be identified. This new paradigm describes how an RNA antitoxin directly interacts with and inhibits a protein toxin. Within this chapter, we discuss the discovery of toxIN as a potent bacteriophage resistance mechanism and the subsequent genetic, biochemical and structural studies that allowed construction of generalised working models for the activities of these novel systems. Furthermore, the recent identification of three independent families of type III systems, suggests the possibility of wider biological roles. Although type III TA loci had perhaps been initially considered a niche field, it is now clear that these systems are widespread, and thus presumably important in the biology of prokaryotes.