Ectopic production of VapCs from Enterobacteria inhibits translation and trans-activates YoeB mRNA interferase

Toxin-antitoxin loci have been identified in almost all free-living prokaryotes, often in high copy numbers. The biological function and molecular targets of the abundant vapBC loci are not yet known. Here we analyse the vapBC loci of Salmonella LT2 and Shigella plasmid pMYSH6000. Both loci encode putative PIN (PilT N-terminal) domain toxins, and antitoxins that may regulate vapBC transcription. We show that vapBC(LT2) and vapBC(pMYSH) are bona fide TA loci: (i) both VapCs inhibited cell growth very efficiently and were counteracted by the cognate VapBs; (ii) both VapCs inhibited translation; (iii) transcription of the vapBC loci was induced by amino acid starvation and chloramphenicol, consistent with the proposal that VapB is an unstable inhibitor of vapBC transcription; (iv) ectopic expression of both VapCs induced a bacteriostatic condition that could be reversed by the cognate antitoxins. Unexpectedly, induction of vapC in Escherichia coli resulted in mRNA cleavage at stop-codons. Surprisingly, these cleavages depended on the yefM yoeB locus, indicating cross-activation between different toxins, that is, VapC activated YoeB mRNA interferase. Activation of YoeB depended on Lon, indicating that Lon degrades YefM antitoxin. Based on these results we present a model that explains activation of YoeB.