Interference of mRNA function by sequence-specific endoribonuclease PemK

In Escherichia coli, programmed cell death is mediated through the system called "addiction module," which consists of a pair of genes encoding a stable toxin and a labile antitoxin. The pemI-pemK system is an addiction module present on plasmid R100. It helps to maintain the plasmid by post-segregational killing in E. coli population. Here we demonstrate that purified PemK, the toxin encoded by the pemI-pemK addiction module, inhibits protein synthesis in an E. coli cell-free system, whereas the addition of PemI, the antitoxin against PemK, resumes the protein synthesis. Further studies reveal that PemK is a sequence-specific endoribonuclease that cleaves mRNAs to inhibit protein synthesis, whereas PemI blocks the endoribonuclease activity of PemK. PemK cleaves only single-stranded RNA preferentially at the 5' or 3' side of the A residue in the "UAH" sequences (where H is C, A, or U). Upon induction, PemK cleaves cellular mRNAs to effectively block protein synthesis in E. coli. The pemK homologue genes have been identified on the genomes of a wide range of bacteria. We propose that PemK and its homologues form a novel endoribonuclease family that interferes with mRNA function by cleaving cellular mRNAs in a sequence-specific manner.